Want to know the technical information of rectifier diodes? The China Electronics Business Alliance provides the latest technical information on rectifier diodes.
The latest knowledge of rectifier diodes introduces IC introduction topics
First, the problem of traditional diode rectification
In recent years, the development of technology has made the operating voltage of the circuit lower and lower, and the current is getting larger and larger. Low-voltage operation helps to reduce the overall power consumption of the circuit, but it also presents new challenges to the design.
Power Technology The loss of the switching power supply is mainly composed of three parts: the loss of the power switching tube, the loss of the technical high-frequency transformer, and the loss of the rectifier at the output end. In the case of low voltage and high current output, the conduction voltage drop of the rectifier diode is high, and the loss of the rectifier at the output end is particularly prominent. (FRD) or ultra-fast recovery diode (SRD) can reach 1.0-1.2V, even with a reduced Schottky diode (SBD), it will produce a voltage drop of about 0.6V, which leads to increased rectification losses, power efficiency reduce.
For example, laptops currently use 3.3V or even 1.8V or 1.5V supply voltages, and consume up to 20A. At this point, the rectification loss of the ultrafast recovery diode is close to or even exceeds 50% of the power output. Even with Schottky diodes, the loss on the rectifier will reach (18% to 40%) PO, accounting for more than 60% of the total power loss. Therefore, the traditional diode rectification circuit can not meet the needs of achieving high efficiency and small volume of low-voltage, high-current switching power supply, and becomes a bottleneck restricting the efficiency of the DC/DC converter.
Second, the basic circuit structure of synchronous rectification
Synchronous rectification is a new technology that uses a very low on-state dedicated power MOSFET to replace the rectifier diode to reduce rectification losses. It can greatly improve the efficiency of the DC/DC converter and there is no dead zone voltage caused by the Schottky barrier voltage. Power MOSFETs are voltage-controlled devices that have a linear relationship in volt-ampere characteristics during turn-on. When using a power MOSFET, it is required that the gate voltage must be synchronized with the phase of the rectified voltage to complete the rectification function, so it is called synchronous rectification.
1. Basic transformer tapping mode double-ended self-excited, isolated buck synchronous rectification circuit
2, single-ended self-excited, isolated buck synchronous rectifier circuit
Figure 1 Basic schematic diagram of single-ended buck synchronous rectifier
The basic principle is shown in Figure 1. V1 and V2 are power MOSFETs. During the positive half cycle of the secondary voltage, V1 is turned on, V2 is turned off, and V1 is turned on. At the negative half of the secondary voltage, V1 is turned off, V2 is turned off. Pass, V2 plays a freewheeling role. The power loss of the synchronous rectification circuit mainly includes the conduction loss of V1 and V2 and the gate drive loss. When the switching frequency is lower than 1MHz, the conduction loss is dominant; when the switching frequency is higher than 1MHz, the gate driving loss is dominant.
3, half bridge other excitation, double current synchronous rectifier circuit
Figure 2 Basic schematic diagram of single-ended buck synchronous rectifier
The basic features of this circuit are:
1) The secondary side of the transformer only needs one winding. Compared with the middle tap structure, the number of secondary windings is only half of that of the intermediate tap structure, so the power loss on the secondary side is relatively small;
2) The output has two filters, and the currents on the two filter inductors add up to obtain the output load current, and the current ripples on the two inductors cancel each other out, so that a small output current pattern is finally obtained. wave;
3) The average current flowing through each filter inductor is only half of the output current, and the loss on the output filter inductor is significantly reduced compared to the intermediate tap structure;
4) There is less high current inter-connection. In the current doubler rectification topology, its secondary high current connection line has only 2 channels, while in the middle tap topology there are 3 channels;
5) Dynamic response is very good.
Its only drawback is that it requires two output filter inductors, which are relatively larger in volume. However, there is a method called integrated magnetic that integrates both of its output filter inductors and transformers into the same core, which greatly reduces the size of the converter.
Third, the circuit case analysis
Design of 16.5W Synchronous Rectifier DC/DC Power Converter
The following introduces a forward and isolated 16.5WDC/DC power converter, which uses DPA-Switch series single-chip switch type DPA424R, DC input voltage range is 36~75V, output voltage is 3.3V, output current is 5A, output The power is 16.5W. With 400kHz synchronous rectification technology, the loss of the rectifier is greatly reduced. When the DC input voltage is 48V, the power efficiency is η=87%. The converter has perfect protection functions, including over/under voltage protection, output overload protection, open loop fault detection, over temperature protection, automatic restart function, and can limit peak current and peak voltage to avoid output overshoot.
The circuit of the 16.5W synchronous rectification DC/DC power converter composed of DPA424R is shown in Fig. 6. The circuit design can be greatly simplified compared to a power converter composed of discrete components. The electromagnetic interference (EMI) filter, which is composed of C1, L1 and C2, is used to filter out electromagnetic interference introduced by the power grid. R1 is used to set the undervoltage value (UUV) and overvoltage value (UOV). When R1=619kΩ, UUV=619kΩ×50μA+2.35V=33.3V, UOV=619kΩ×135μA+2.5V=86.0V. R1 also linearly reduces the maximum duty cycle when the input voltage is too high, preventing magnetic saturation. R3 is the limit current setting resistor. When R3=11.1kΩ is taken, the set drain limit current I'LIMIT=0.6ILIMIT=0.6×2.50A=1.5A. The voltage regulator VDZ1 (SMBJ150) in the circuit clamps the drain voltage to ensure magnetic reset of the high-frequency transformer.
Figure 6 16.5W synchronous rectification DC/DC power converter circuit
The power supply uses a SI4800 power MOSFET with a very low drain-source on-state resistance as a rectifier. Its maximum drain-source voltage UDS(max)=30V, maximum gate-source voltage UGS(max)=±20V, maximum drain current It is 9A (25°C) or 7A (70°C) with a peak drain current of 40A and a maximum power dissipation of 2.5W (25°C) or 1.6W (70°C). SI4800's on-time tON=13ns (including on-delay time td(ON)=6ns, rise time tR=7ns), turn-off time tOFF=34ns (including turn-off delay time td(OFF)=23ns, fall time tF =11ns), transconductance gFS = 19S. The operating temperature range is -55 to +150 °C. Inside the SI4800, there is a freewheeling diode VD, which is connected in parallel with the reverse polarity between the drain and the source (the negative terminal is connected to D and the positive terminal is connected to S), which can protect the MOSFET power tube. The reverse recovery time of the VD is trr=25 ns. [~pvp~]
The power MOSFET is different from the bipolar transistor technology transistor in that its gate CGS is large, and the CGS is first charged before being turned on, only when the voltage on the CGS exceeds the gate-source turn-on voltage [UGS(th)]. The MOSFET begins to conduct. For the SI4800, UGS(th) ≥ 0.8V. To ensure that the MOSFET is turned on, the UGS used to charge the CGS is higher than the nominal value, and the equivalent gate capacitance is many times higher than the CGS.
The relationship between the gate-source voltage (UGS) of the SI4800 and the total gate charge (QG) is shown in Figure 7. As can be seen from Figure 7
QG=QGS+QGD+QOD(1)
Where: QGS is the gate-source charge;
QGD is the gate-drain charge, also known as the charge on the Miller capacitor;
QOD is the overcharge of the Miller capacitor.
Figure 7 shows the relationship between UGS and QG of SI4800
When UGS=5V, QGS=2.7nC, QGD=5nC, QOD=4.1nC, it is not difficult to calculate in the substitution formula (1), and the total gate charge QG=11.8nC.
The equivalent gate capacitance CEI is equal to the total gate charge divided by the gate-source voltage, ie
CEI=QG/UGS(2)
Substituting QG=11.8nC and UGS=5V into equation (2), the equivalent gate capacitance CEI=2.36nF can be calculated. It should be noted that the equivalent gate capacitance is much larger than the actual gate capacitance (ie, CEI > CGS). Therefore, the gate peak drive current IG (PK) required for conduction within a specified time should be calculated in terms of CEI. IG(PK) is equal to the total gate charge divided by the on-time, ie
IG=QG/tON(3)
Substituting QG=11.8nC and tON=13ns into equation (3), IG(PK)=0.91A required for conduction can be calculated.
Synchronous rectifier V2 is driven by a secondary voltage and R2 is the gate load of V2. The synchronous freewheeling tube V1 is directly driven by the reset voltage of the high frequency transformer, and V1 operates only when V2 is turned off. When the Schottky diode VD2 is turned off, a portion of the energy is stored on the common mode choke L2. When the high-frequency transformer completes the reset, VD2 continues to conduct, and the energy in L2 continues to supply power to the load through VD2, maintaining the output voltage unchanged. The output of the auxiliary winding is rectified and filtered by VD1 and C4 to provide a bias voltage to the receiving tube in the optocoupler. C5 is the bypass capacitor of the control terminal. The time for power-on and auto-restart is determined by C6.
After the output voltage is divided by R10 and R11, it is compared with the 2.50V reference voltage in the adjustable precision shunt regulator LM431 to generate the error voltage. Then the optocoupler PC357 is used to control the duty cycle of the DPA424R. adjust. R7, VD3 and C3 form a soft-start circuit to avoid overshoot of the output voltage when the power is first turned on. At the time of power-on, the LM431 does not work because the voltage across C3 cannot be abruptly changed. As the output voltage of the rectifier filter rises and charges C3 through R7, the voltage on C3 rises and the LM431 turns into normal operation. During the soft start process, the output voltage rises slowly and eventually reaches a stable value of 3.3V.
Fourth, the latest developments in power MOSFETs for synchronous rectification
In order to meet the needs of high-frequency, large-capacity synchronous rectification circuits, some dedicated power MOSFETs have been introduced in recent years. Typical NDS8410 N-channel power MOSFETs manufactured by FAIRCHILD have an on-state resistance of 0.015 Ω. The SI4800 power MOSFET manufactured by Philips is manufactured using TrenchMOSTM technology. The on and off states can be controlled by logic levels, and the drain-source on-state resistance is only 0.0155Ω. IR's IRL3102 (20V/61A), IRL2203S (30V/116A), and IRL3803S (30V/100A) power MOSFETs have on-resistances of 0.013Ω, 0.007Ω, and 0.006Ω, respectively, when passing 20A. The turn-on voltage drop is less than 0.3V. These dedicated power MOSFETs have high input impedance and short switching times, making them the preferred rectifier devices for designing low-voltage, high-current power converters.
Recently, foreign IC manufacturers have also developed synchronous rectification (SRIC). For example, IR's recently introduced IR1176 is a high-speed CMOS specifically designed to drive N-channel power MOSFETs. The IR1176 can operate independently of the primary side topology and does not require the addition of complex circuits such as active clamps and gate drive compensation. The IR1176 is suitable for synchronous rectifiers in high current DC/DC converters with output voltages below 5V, which greatly simplifies and improves the design of isolated DC/DC converters in broadband network servers. The IR1176 is equipped with an IRF7822 power MOSFET to increase the efficiency of the converter. When the input voltage is +48V and the output is +1.8V, 40A, the efficiency of the DC/DC converter can reach 86%, and the efficiency at the output of 1.5V can still reach 85%.
4 Conclusion
When designing a low-voltage, high-current output DC/DC converter, synchronous rectification technology can significantly improve power efficiency. When driving a high-power synchronous rectifier, when the gate peak drive current IG(PK) ≥ 1A is required, a CMOS high-speed power MOSFET driver such as TC4426A to TC4428A developed by Microchip can be used.
The latest knowledge of rectifier diodes introduces IC introduction topics
First, the problem of traditional diode rectification
In recent years, the development of technology has made the operating voltage of the circuit lower and lower, and the current is getting larger and larger. Low-voltage operation helps to reduce the overall power consumption of the circuit, but it also presents new challenges to the design.
Power Technology The loss of the switching power supply is mainly composed of three parts: the loss of the power switching tube, the loss of the technical high-frequency transformer, and the loss of the rectifier at the output end. In the case of low voltage and high current output, the conduction voltage drop of the rectifier diode is high, and the loss of the rectifier at the output end is particularly prominent. (FRD) or ultra-fast recovery diode (SRD) can reach 1.0-1.2V, even with a reduced Schottky diode (SBD), it will produce a voltage drop of about 0.6V, which leads to increased rectification losses, power efficiency reduce.
For example, laptops currently use 3.3V or even 1.8V or 1.5V supply voltages, and consume up to 20A. At this point, the rectification loss of the ultrafast recovery diode is close to or even exceeds 50% of the power output. Even with Schottky diodes, the loss on the rectifier will reach (18% to 40%) PO, accounting for more than 60% of the total power loss. Therefore, the traditional diode rectification circuit can not meet the needs of achieving high efficiency and small volume of low-voltage, high-current switching power supply, and becomes a bottleneck restricting the efficiency of the DC/DC converter.
Second, the basic circuit structure of synchronous rectification
Synchronous rectification is a new technology that uses a very low on-state dedicated power MOSFET to replace the rectifier diode to reduce rectification losses. It can greatly improve the efficiency of the DC/DC converter and there is no dead zone voltage caused by the Schottky barrier voltage. Power MOSFETs are voltage-controlled devices that have a linear relationship in volt-ampere characteristics during turn-on. When using a power MOSFET, it is required that the gate voltage must be synchronized with the phase of the rectified voltage to complete the rectification function, so it is called synchronous rectification.
1. Basic transformer tapping mode double-ended self-excited, isolated buck synchronous rectification circuit
2, single-ended self-excited, isolated buck synchronous rectifier circuit
Figure 1 Basic schematic diagram of single-ended buck synchronous rectifier
The basic principle is shown in Figure 1. V1 and V2 are power MOSFETs. During the positive half cycle of the secondary voltage, V1 is turned on, V2 is turned off, and V1 is turned on. At the negative half of the secondary voltage, V1 is turned off, V2 is turned off. Pass, V2 plays a freewheeling role. The power loss of the synchronous rectification circuit mainly includes the conduction loss of V1 and V2 and the gate drive loss. When the switching frequency is lower than 1MHz, the conduction loss is dominant; when the switching frequency is higher than 1MHz, the gate driving loss is dominant.
3, half bridge other excitation, double current synchronous rectifier circuit
Figure 2 Basic schematic diagram of single-ended buck synchronous rectifier
The basic features of this circuit are:
1) The secondary side of the transformer only needs one winding. Compared with the middle tap structure, the number of secondary windings is only half of that of the intermediate tap structure, so the power loss on the secondary side is relatively small;
2) The output has two filters, and the currents on the two filter inductors add up to obtain the output load current, and the current ripples on the two inductors cancel each other out, so that a small output current pattern is finally obtained. wave;
3) The average current flowing through each filter inductor is only half of the output current, and the loss on the output filter inductor is significantly reduced compared to the intermediate tap structure;
4) There is less high current inter-connection. In the current doubler rectification topology, its secondary high current connection line has only 2 channels, while in the middle tap topology there are 3 channels;
5) Dynamic response is very good.
Its only drawback is that it requires two output filter inductors, which are relatively larger in volume. However, there is a method called integrated magnetic that integrates both of its output filter inductors and transformers into the same core, which greatly reduces the size of the converter.
Third, the circuit case analysis
Design of 16.5W Synchronous Rectifier DC/DC Power Converter
The following introduces a forward and isolated 16.5WDC/DC power converter, which uses DPA-Switch series single-chip switch type DPA424R, DC input voltage range is 36~75V, output voltage is 3.3V, output current is 5A, output The power is 16.5W. With 400kHz synchronous rectification technology, the loss of the rectifier is greatly reduced. When the DC input voltage is 48V, the power efficiency is η=87%. The converter has perfect protection functions, including over/under voltage protection, output overload protection, open loop fault detection, over temperature protection, automatic restart function, and can limit peak current and peak voltage to avoid output overshoot.
The circuit of the 16.5W synchronous rectification DC/DC power converter composed of DPA424R is shown in Fig. 6. The circuit design can be greatly simplified compared to a power converter composed of discrete components. The electromagnetic interference (EMI) filter, which is composed of C1, L1 and C2, is used to filter out electromagnetic interference introduced by the power grid. R1 is used to set the undervoltage value (UUV) and overvoltage value (UOV). When R1=619kΩ, UUV=619kΩ×50μA+2.35V=33.3V, UOV=619kΩ×135μA+2.5V=86.0V. R1 also linearly reduces the maximum duty cycle when the input voltage is too high, preventing magnetic saturation. R3 is the limit current setting resistor. When R3=11.1kΩ is taken, the set drain limit current I'LIMIT=0.6ILIMIT=0.6×2.50A=1.5A. The voltage regulator VDZ1 (SMBJ150) in the circuit clamps the drain voltage to ensure magnetic reset of the high-frequency transformer.
Figure 6 16.5W synchronous rectification DC/DC power converter circuit
The power supply uses a SI4800 power MOSFET with a very low drain-source on-state resistance as a rectifier. Its maximum drain-source voltage UDS(max)=30V, maximum gate-source voltage UGS(max)=±20V, maximum drain current It is 9A (25°C) or 7A (70°C) with a peak drain current of 40A and a maximum power dissipation of 2.5W (25°C) or 1.6W (70°C). SI4800's on-time tON=13ns (including on-delay time td(ON)=6ns, rise time tR=7ns), turn-off time tOFF=34ns (including turn-off delay time td(OFF)=23ns, fall time tF =11ns), transconductance gFS = 19S. The operating temperature range is -55 to +150 °C. Inside the SI4800, there is a freewheeling diode VD, which is connected in parallel with the reverse polarity between the drain and the source (the negative terminal is connected to D and the positive terminal is connected to S), which can protect the MOSFET power tube. The reverse recovery time of the VD is trr=25 ns. [~pvp~]
The power MOSFET is different from the bipolar transistor technology transistor in that its gate CGS is large, and the CGS is first charged before being turned on, only when the voltage on the CGS exceeds the gate-source turn-on voltage [UGS(th)]. The MOSFET begins to conduct. For the SI4800, UGS(th) ≥ 0.8V. To ensure that the MOSFET is turned on, the UGS used to charge the CGS is higher than the nominal value, and the equivalent gate capacitance is many times higher than the CGS.
The relationship between the gate-source voltage (UGS) of the SI4800 and the total gate charge (QG) is shown in Figure 7. As can be seen from Figure 7
QG=QGS+QGD+QOD(1)
Where: QGS is the gate-source charge;
QGD is the gate-drain charge, also known as the charge on the Miller capacitor;
QOD is the overcharge of the Miller capacitor.
Figure 7 shows the relationship between UGS and QG of SI4800
When UGS=5V, QGS=2.7nC, QGD=5nC, QOD=4.1nC, it is not difficult to calculate in the substitution formula (1), and the total gate charge QG=11.8nC.
The equivalent gate capacitance CEI is equal to the total gate charge divided by the gate-source voltage, ie
CEI=QG/UGS(2)
Substituting QG=11.8nC and UGS=5V into equation (2), the equivalent gate capacitance CEI=2.36nF can be calculated. It should be noted that the equivalent gate capacitance is much larger than the actual gate capacitance (ie, CEI > CGS). Therefore, the gate peak drive current IG (PK) required for conduction within a specified time should be calculated in terms of CEI. IG(PK) is equal to the total gate charge divided by the on-time, ie
IG=QG/tON(3)
Substituting QG=11.8nC and tON=13ns into equation (3), IG(PK)=0.91A required for conduction can be calculated.
Synchronous rectifier V2 is driven by a secondary voltage and R2 is the gate load of V2. The synchronous freewheeling tube V1 is directly driven by the reset voltage of the high frequency transformer, and V1 operates only when V2 is turned off. When the Schottky diode VD2 is turned off, a portion of the energy is stored on the common mode choke L2. When the high-frequency transformer completes the reset, VD2 continues to conduct, and the energy in L2 continues to supply power to the load through VD2, maintaining the output voltage unchanged. The output of the auxiliary winding is rectified and filtered by VD1 and C4 to provide a bias voltage to the receiving tube in the optocoupler. C5 is the bypass capacitor of the control terminal. The time for power-on and auto-restart is determined by C6.
After the output voltage is divided by R10 and R11, it is compared with the 2.50V reference voltage in the adjustable precision shunt regulator LM431 to generate the error voltage. Then the optocoupler PC357 is used to control the duty cycle of the DPA424R. adjust. R7, VD3 and C3 form a soft-start circuit to avoid overshoot of the output voltage when the power is first turned on. At the time of power-on, the LM431 does not work because the voltage across C3 cannot be abruptly changed. As the output voltage of the rectifier filter rises and charges C3 through R7, the voltage on C3 rises and the LM431 turns into normal operation. During the soft start process, the output voltage rises slowly and eventually reaches a stable value of 3.3V.
Fourth, the latest developments in power MOSFETs for synchronous rectification
In order to meet the needs of high-frequency, large-capacity synchronous rectification circuits, some dedicated power MOSFETs have been introduced in recent years. Typical NDS8410 N-channel power MOSFETs manufactured by FAIRCHILD have an on-state resistance of 0.015 Ω. The SI4800 power MOSFET manufactured by Philips is manufactured using TrenchMOSTM technology. The on and off states can be controlled by logic levels, and the drain-source on-state resistance is only 0.0155Ω. IR's IRL3102 (20V/61A), IRL2203S (30V/116A), and IRL3803S (30V/100A) power MOSFETs have on-resistances of 0.013Ω, 0.007Ω, and 0.006Ω, respectively, when passing 20A. The turn-on voltage drop is less than 0.3V. These dedicated power MOSFETs have high input impedance and short switching times, making them the preferred rectifier devices for designing low-voltage, high-current power converters.
Recently, foreign IC manufacturers have also developed synchronous rectification (SRIC). For example, IR's recently introduced IR1176 is a high-speed CMOS specifically designed to drive N-channel power MOSFETs. The IR1176 can operate independently of the primary side topology and does not require the addition of complex circuits such as active clamps and gate drive compensation. The IR1176 is suitable for synchronous rectifiers in high current DC/DC converters with output voltages below 5V, which greatly simplifies and improves the design of isolated DC/DC converters in broadband network servers. The IR1176 is equipped with an IRF7822 power MOSFET to increase the efficiency of the converter. When the input voltage is +48V and the output is +1.8V, 40A, the efficiency of the DC/DC converter can reach 86%, and the efficiency at the output of 1.5V can still reach 85%.
4 Conclusion
When designing a low-voltage, high-current output DC/DC converter, synchronous rectification technology can significantly improve power efficiency. When driving a high-power synchronous rectifier, when the gate peak drive current IG(PK) ≥ 1A is required, a CMOS high-speed power MOSFET driver such as TC4426A to TC4428A developed by Microchip can be used.
Shenzhen Ever-smart Sensor Technology Co., LTD , https://www.fluhandy.com