Today, high-frequency practical switching power supplies with rated powers of more than 200W are more or less troublesome in terms of environmental performance evaluation. They either have large EMI noise, or the input current harmonics are excessive or the temperature characteristics are not good at a certain power package density, and the reliability is poor. One way to solve these problems is to find new and more advanced converter topologies. Another way is to choose new processes and new devices to meet environmental performance assessment requirements.
In recent years, some well-known foreign semiconductor companies have spent a lot of effort to transform the device technology and develop a series of targeted new devices with superior performance. For example, Infineon, formerly known as Siemens, has successively launched a package of devices dedicated to solving the above problems of high-frequency switching power supplies. They include a high voltage 600V, low on-resistance (RDSOn) CoolMOS tube (very low temperature rise during high frequency operation, suitable for Boost switch), high current, low withstand voltage and small Rdson OptiMOS tube (extraordinarily suitable for Buck converter) ), PFC-PWM dual-in-one ICTDA16888 (saving space and components), high voltage (600V) SiC Schottky diode (especially for Boost diodes) and so on. These devices have special features that, if used properly in switching power supply design, will solve the problem with less effort and cost.
As an example, this article intends to introduce to the reader a practical switching power supply that meets environmental requirements with a working frequency of 200 kHz and a power of 200 W. It uses the second generation of CoolMOSC2 as the PFC and PWM power switch, SiC Schottky diode as the PFC diode, OptiMOS as the synchronous rectification switch, PFC and PWM control by the same ICTDA16888. The power supply has a wide input voltage range (90V ~ 275V), 80% or more AC / DC conversion efficiency. There are two sets of output voltages: +5V/20A and +12V/8.3A with output overload protection and output short-circuit protection. All power devices do not require a heat sink and do not require a minimum output load.
2 circuit block diagram
Figure 1 shows the operational block diagram of the overall power supply. It is composed of two parts: PFC and PWM. The first part is an AC/DC converter for power factor correction (PFC), and the second part is a forward pulse width modulation (PWM) DC/DC converter consisting of two power switches. The PFC stage is a Boost boost converter that provides a 380Vd.c. at its output while maintaining the input current as a sine wave at the input to achieve a power factor approximately equal to one. Another feature of the PFC stage is that it allows the power supply to operate over a wide voltage input range (90V to 275V) without the need to add a voltage range switch that reconfigures the rectifier circuit. The power devices used were two CoolMOS SPB11N60C2 in parallel and one SiC Schottky diode SDB06S60 (6A/600V).
The two-switch forward converter is isolated from the grid by a coupling transformer T1. At the transformer primary, the power devices are two CoolMOS SPB11N60C2 and two EMCON diodes SDD04E60 (4A/600V). The secondary has two sets of outputs (5Vd.c. and 12Vd.c.), but their rectification principles are different. The 12V output uses a conventional Schottky diode rectifier circuit, while the 5V output uses a low voltage MOSFET SSPB80N03S2L-03 for synchronous rectification.
The functional control of both PFC and PWM is done by a single integrated circuit TDA16888.
3 structure / heat sink design
One of the advantages of this power supply is its small size. It consists of two double-sided PCB boards of different sizes. The larger piece (18cm × 15cm) is the main board, equipped with various power devices and passive components, and try to use small SMD components. The device does not use any heat sink, and the heat is dissipated by the main copper skin on the PCB to transfer thermal energy to the underlying metal plate. The smaller piece (6 cm x 3 cm) is the control board, which is equipped with a control circuit and is inserted vertically into the main board.
4 component function description
4.1 Power Motherboard
The principle of the power supply board is shown in Figure 2, including the following parts:
(1) AC input / EMI filter
The input voltage of the SMPS is 90V to 275V (50Hz/60Hz). The fuse is used to prevent further damage to the power supply in the event of a circuit failure. The input EMI filters (C86, L1, L4, C24, C25, C26, C2) are used to suppress high frequency noise generated when switching between the two power switches. The varistor R30 is used to withstand high voltage surges originating from the power grid. The input power rectifier (D1–D4) uses a conventional silicon diode.
(2) PFC converter
This is a Boost boost converter topology with continuous inductor current flowing through the full load. The switching frequency is 200kHz. The output voltage is approximately 380Vd.c.
The core part of the PFC is the Boost inductor L2, the switching transistor Q1A/Q1B, the boost diode D5 and the large electrolytic capacitor C3. In order to reduce the parasitic capacitance, L2 is formed by winding a single copper wire on a toroidal iron powder core. Parallel tubes Q1A/Q1B are SPB11N60C2 made with the new CoolMOS process. They have high switching speed and very low on-state resistance. This advantage is due to the high current and high duty cycle of the circuit at 90V low input. So it is especially important. The purpose of the parallel connection of the two tubes is simply to expand the heat dissipation area in order to balance the heat distribution on the PCB. Boost diode D5 is a 600V SiC Schottky diode with very good switching characteristics due to its lack of charge storage (no reverse recovery and no temperature disturbance to switching characteristics). The D82 is a conventional silicon diode used to charge the electrolytic capacitor from the initial rectified voltage to prevent the SiC diode D5 from being subjected to excessive surge currents at the moment of startup. Electrolytic capacitor C3 is used to store energy to reduce the voltage ripple of the second harmonic, and it must also withstand the switching frequency current. Capacitor C3A is dedicated to bypassing high frequency harmonic currents.
Figure 5: Conducted noise test
(a) AV average detection noise spectrum line (b) QP quasi-peak detection noise line
Furthermore, since the PWM uses an optocoupler and a variable voltage regulator ICTL431 as an output voltage feedback feedback circuit, its load-regulation regulation rate is also very good, and there is no need to impose additional requirements on the load size in order to obtain a stable output voltage.
5.2 Distribution of power consumption
Maximum power consumption occurs at full load and low AC input voltage conditions. The operating point at this time is: Vin=90V, PIN=224W, Pout=180.5W, power consumption Ploss=43.5W
The distribution of power consumption can be estimated by using the temperature of the component under test as shown in Table 2.
Table 2 power distribution
In order to test the conduction noise of the whole switching power supply, we test it according to the test method of EMI noise specified by CISPRPublication16, 1977, using EMI receiver FMLK1518 and a power supply impedance stabilization network (LISN) NSLK8128. The result is shown in Fig. 5. Test conditions: Vin=230V, Pout=181.4W, the whole machine power is placed in the metal box.
As can be seen from Figure 5, the measured EMI noise lines are below the normal limits.
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