PFC’S full name is “Factor Correction Power”, which means “power factor correction”. The power factor refers to the relationship between the effective power and the total power consumption (apparent power). The power factor can be used to measure the power of the electric power, and when the power factor is higher, the higher the power utilization ratio is. With PFC switching power supply costs are relatively high.
Switching power supply is a kind of capacitor input circuit, the phase difference between the current and voltage can cause the loss of the switching power, which requires the PFC circuit to improve the power factor. At present, there are two kinds of PFC, passive PFC (also known as passive PFC) and active PFC (also known as PFC).
Passive PFC is generally divided into “Valley” and “Fill Circuit”. The phase difference between the fundamental current and the voltage of the AC input is reduced to improve the power factor, and the passive PFC includes the silent type passive PFC and passive passive PFC. The power factor of the passive PFC can only reach 0.7 ~ 0.8, it is generally in the vicinity of the high voltage filter capacitor.
The valley fill circuit belongs to a new type of passive power factor correction circuit and its characteristics is the rectifier bridge behind the valley fill circuit to greatly increase rectifier conduction angle, through filled Valley, the current input from spike pulse becomes close to sinusoidal waveform, power factor is increased to 0.9%, significantly reduce the total harmonic distortion. Compared with the traditional passive power factor correction circuit, the circuit is simple, the power factor compensation is remarkable, and it is not required to use the large size of the large weight of the input circuit.
The active PFC is composed of the inductor capacitor and electronic components, small size, through the special IC to adjust the current waveform, the phase difference between the current and voltage compensation. Active PFC can achieve high power factor, which is usually more than 98%, but the cost is relatively high. In addition, the active PFC can also be used as auxiliary power supply, so in the use of active PFC circuit, it is often not necessary to standby transformer, and active PFC output DC voltage ripple is very small, this power does not need to use a large capacity of filter capacitor
The power module system structure diagram as shown in Figure 220, it can be seen that the 1 V AC voltage signal input, first through the filter circuit module, and then sub channels to achieve AC DC voltage signal, through the DC DC/ converter is 12 V and V 10:1 DC voltage signal, and then through the V converter to get 23 +9 DC voltage signal, and the use of DC voltage integrated voltage regulator V +12 and V +9 voltage, V +5 as a reference signal, while the control circuit to provide a positive voltage. Control circuit is mainly divided into control circuit and over voltage protection circuit, control circuit is mainly used to achieve the control of the output voltage of the control, and over voltage protection circuit is mainly used to achieve the protection of over voltage, play a necessary role in the protection of three DC/DC.
1) design objectives.
The design target of the module is AC/DC power supply module, the input voltage is 220 Hz V/50 AC input, the output DC voltage is 12 V, V +5, V +9 and V. +6
(2) filtered rectifier circuit.
In order to filter out the interference in the circuit, the power input is 410-3/02 SCHAFFNER, the rated current of the filter is 3 A, the maximum operating voltage is 250 V, the frequency is Hz -25, the operating temperature is ~+100, and the average time is 675000 hours. In this power module, the filter is needed for each voltage regulator module, reference source and the output terminal of the DC voltage, so the selection of the electrolytic capacitor and electrolytic capacitor value is from 47 V to 1000 V F/16.
Rectifier bridge is selected according to the different of the rectifier circuit is divided into 220 kinds, one is the two V AC rectifier for 300 V DC circuit, the use of KBPC 108 rectifier bridge, the input voltage of V 50~1000, the input current is 3 A, used to achieve high voltage rectifier.
The other is low voltage rectification, in this circuit, the first is the 220V after the 10:1 AC power transformer, the use of rectifier bridge rectifier, the output DC voltage of 23 V.
(3) DC/DC circuit design.
In order to obtain a stable and reliable 12 V and V +5 DC voltage, the high voltage DC output of the DC/DC module is implemented in the DC/DC circuit. At the low voltage side, the V and V +9 output are achieved by using 23 V F/25 and +12 output at the input and output of each module, and the electrolytic capacitor is filtered by 100 V F/25 and 47 V respectively. At the high voltage side, three + 12 V and V +5 DC voltage are generated, and the output of the three voltage signals can be controlled by the external interface. Therefore, the VI-J61-IZ, VI-J61-IY and VI-J60-IX power supply module of VICOR is used to realize the output of 12 V and V +5. The power input of the three modules is connected with 300 V DC power supply, and the high precision of 12 V and V +5 voltage is obtained. The output control of DC/DC is required, and the In Gate of the three power modules is controlled, and the schematic diagram of the three DC/DC circuit is shown in Figure 2. In Figure 2, when the control signal is high, VT1, VT2 and VT3 work, the DC/DC 2 is not working, DC/DC and +5V are not working, and the control signal is low, VT2, VT3 and VT1 are not working, at this time DC/DC are normal, 12 V +5 and V 12V voltage output.
(4) DC voltage control circuit.
The schematic diagram of the DC voltage control circuit is shown in figure 3. The circuit is composed of two parts, which are over voltage protection circuit and the external voltage control circuit. Over voltage protection circuit mainly refers to when the input voltage is too high (or low) to produce more than (less than) 300 V after a certain percentage of the voltage, after conditioning circuit to make the voltage comparator MAX973 voltage jump, so as to change the output of the control signal, resulting in the In Gate DC/DC terminal voltage jump, and then make DC/DC stop working. The external voltage control circuit refers to the output voltage of the output terminal of the control signal changes when the external control signal input terminal is changed, thereby changing the voltage of the Gate In DC/DC, making the DC/DC stop (or start) work.
When the external control signal input is low at ordinary times, NAND circuit trigger output to a high level, at this time the counter is cleared, after counting trigger circuit and the inverter inverting control signal output for the high level so as to further verify the three DC-DC does not work, the corresponding DC / DC working indicator light does not shine. When the external control signal input is high at ordinary times, NAND circuit trigger output to a low level. At this time, the counter starts to count, after counting trigger circuit and the inverter inverting control signal output low level, so as to further verify the three DC-DC normal work and + 12 V and + 5 V output voltage, DC / DC indicator lights.
In the electroplating industry, the general requirements of the power supply of the output voltage is low, and the current is very big. Power requirements are relatively high, the general is thousands of watts to tens of kilowatts. At present, such a large power of the electroplating power is generally used thyristor phase controlled rectifier. The disadvantages are the large size, low efficiency, high noise, low power factor, large output ripple, slow dynamic response, poor stability and so on.
In this paper, the switching power supply is introduced, and the output voltage from 0 to 12V and the current from 0 to 5000A can be adjusted continuously, and full load output power is 60kW. Due to the use of ZVT soft switch technology, and the use of a better cooling structure, the power of the indicators have met the requirements of the user, has now been put into production in small quantities.
2 main circuit topology
In view of such a high power output, high frequency inverter part adopts IGBT as power switching device of the full bridge topology, the main circuit as shown in Figure 1, including: power frequency three phase AC input, diode rectifier bridge, EMI filter, filter inductance capacitance, high frequency full bridge inverter, high frequency transformer, output rectifier, output LC filter, etc..
The DC capacitor Cb is used to balance the power of the transformer to the second value and prevent the bias. In consideration of the efficiency of the problem, the resonant inductor Ls only uses the leakage inductance of the transformer itself. Because if the inductor is too large, it will lead to high off voltage spike, which is very bad for the switch tube, but also increase the off loss. On the other hand, it can also cause serious duty cycle loss, which causes the current peak value of the switch device, which makes the performance of the system decrease.
3 zero voltage soft switch
High frequency full bridge inverter control mode for the phase shifted FB-ZVS control mode, control chip using Unitrode UC3875N. In the full load range, the zero voltage soft switching is achieved by the lead arm in the full load range. Figure 2. The driving voltage and the collector emitter voltage waveform of the lagging leg IGBT can be seen to achieve zero voltage turn-on.
Switching frequency selection 20kHz, this design can reduce the IGBT of the off loss, on the other hand, it can take into account the high frequency, so that the power transformer and output filter of the volume decreases.
4 capacitive power bus
In the first experiment, between the connection bus capacitor C5 and IGBT module for common power bus. In the experiment, the voltage and current of IGB on IGBT are all high frequency oscillations, and the primary voltage and current waveform of the transformer are collected in Figure 3. The reason is in parallel with the IGBT module on the surge absorbing parasitic capacitance and inductance power bus has high frequency resonance. One hour after the full load operation, power bus temperature is 38 DEG C, the capacitor C5 temperature is 24 DEG C.
5 a series of parallel transformers are used to realize the parallel structure of the output rectifier diodes.
In order to further reduce the loss, the output rectifier diodes using only current 400A, resistance to high voltage 80V Schottky diode in parallel. Moreover, the secondary output of each transformer uses a full wave rectification method. This sample is only one group of diodes that flow through the current. At the same time, in the secondary rectifier diode with on RC snubber network, to suppress by the transformer leakage and Schottky diode body capacitance caused by the parasitic oscillations. All these measures can reduce the power consumption and improve the efficiency.
For large current output, the general output of the rectifier diode in parallel. But because the Schottky diode is a negative temperature coefficient of the device, in parallel, the general should take into account the flow between them. There are many kinds of parallel modes of the diode, the graph a is the direct parallel mode, the B is a series of parallel connection mode, and the C is a series of dynamic current transformer. (the parallel connection of four diodes).
For the direct parallel method, the diode current is very poor, the output current is generally limited to tens of amps to hundreds of AMPS, not easy to do a thousand amps. In order to achieve the purpose of the current, the current can be used to achieve the purpose of the current flow. Because of the influence of proximity effect and skin effect, the average flow of the diode is changed with the output current, and the effect is poor. In order to achieve a good result, the resistance of the string is not too small, which brings great loss. For the series of dynamic current transformer, the parallel mode can achieve better results, but the production process of large current transformer is complex, the cost is high, and the leakage inductance and the lead inductance of the dynamic current transformer can be increased.
In order to overcome the shortcomings of the above parallel mode, the output rectifier diodes can achieve both automatic current sharing, reduce the loss, and can reduce the complexity of the production process, we have designed a novel high frequency power transformer, as shown in Figure 1. The transformer is composed of eight identical small transformer, the ratio was 4: 1, their primary series, while the secondary is the parallel structure. The transformer uses the primary cooling and secondary cooling method, which is considered to be different, and can greatly simplify the process of the transformer.