“Generally speaking, the power circuit of the power supply mainly includes an input filter capacitor, an output filter capacitor, a filter Inductor, and upper and lower power FETs. The control circuit mainly includes a PWM control chip, a bypass capacitor, a bootstrap circuit, a feedback voltage divider resistor, and a feedback compensation circuit.
Generally speaking, the power circuit of the power supply mainly includes an input filter capacitor, an output filter capacitor, a filter inductor, and upper and lower power FETs. The control circuit mainly includes a PWM control chip, a bypass capacitor, a bootstrap circuit, a feedback voltage divider resistor, and a feedback compensation circuit.
It is a schematic diagram of a switching power supply used in consumer electronics products. The designer should be able to distinguish the components in the power circuit and the components in the control signal circuit on this circuit diagram. If the designer treats all the components in the power supply as components in a digital circuit, the problem will be quite serious. Usually, it is first necessary to know the path of the high-frequency current of the power supply, and to distinguish the small-signal control circuit and the power circuit components and their wiring.
A schematic diagram of a typical step-down switching power supply (12V input, 3.3V/15A output)[power control circuit (thin line), power circuit (thick line)]power circuit PCB layout
The correct placement and routing of power devices on the PCB will determine whether the entire power supply works normally. The designer must first have a certain understanding of the voltage and current waveforms on the switching power supply power device.
The following shows the current and voltage waveforms on the power circuit components of a step-down switching power supply.
Since the current flowing from the input filter capacitor (Cin), the upper FET (S1) and the lower FET (S2) is an AC current with high frequency and high peak value, it is determined by Cin-S1-S2 The area of the loop formed should be minimized. At the same time, the area of the loop formed by S2, L and the output filter capacitor (Cout) should also be minimized.
Current and voltage on the power circuit of the switching power supply
If the designer does not make the power circuit PCB according to the points described in this article, it is likely to make the wrong power supply PCB as shown below.
Incorrect placement and wiring of switching power supply power devices
There are many errors in this PCB layout:
Because Cin has a large ESL, Cin’s high-frequency filtering capability basically disappears; second, the area of the Cin-S1-S2 and S2-L-Cout loop is too large, the electromagnetic noise generated will affect the power supply itself and Peripheral circuits cause a lot of interference; third, the pads of L are too close, causing CP to be too large and reducing its high-frequency filtering function; fourth, the lead of the Cout pad is too long, causing ESL to be too large and lost High frequency filtering function.
This is a better PCB trace for the power supply circuit. The area of the Cin-S1-S2 and S2-L-Cout loops has been controlled.
The connection point between the source of S1, the drain of S2 and L is a whole piece of copper pad. Since the voltage on this connection point is high frequency, S1, S2, and L need to be very close. Although there is no high-peak high-frequency current on the trace between L and Cout, a wider trace can reduce the loss of DC impedance and improve the efficiency of the power supply.
If cost permits, the power supply can be used with a double-sided PCB with a ground layer on one side, but care must be taken to avoid running power and signal lines on the ground layer as much as possible. A ceramic capacitor is added to the input and output ports of the power supply to improve the high-frequency filtering performance of the power supply.
Power control circuit PCB layout
The layout of the power control circuit PCB is also very important. Unreasonable layout will cause drift and oscillation of the output voltage of the power supply. The control circuit should be placed on the side of the power circuit, not in the middle of the high frequency AC loop. The bypass capacitor should be as close as possible to the VCC and ground pin (GND) of the chip. The feedback divider resistor is also placed near the chip. The loop that the chip drives to the FET should also be as short as possible.
The basic points of power layout: The drive circuit loop from the control chip to the upper and lower FETs should be as short as possible.
Switching power supply PCB layout example
The wiring diagram of the typical step-down switching power supply PCB that you saw in the article. A low-cost PWM controller (Semtech model SC1104A) is used in this power supply. The lower layer of the PCB is a complete ground plane. There is no separation between the PCB power ground plane and the control ground plane.
It can be seen that the power circuit of the power supply goes from the input socket (the upper left end of the PCB) through the input filter capacitors (C1, C2), S1, S2, L1, and the output filter capacitors (C10, C11, C12, C13) to the output socket ( PCB bottom right). SC1104A is placed on the lower left end of the PCB.
Because the power circuit current does not pass through the control circuit on the ground layer, it is not necessary to separate the control circuit ground layer from the power circuit ground layer. If the input socket is placed at the lower left end of the PCB, then the power circuit current on the ground layer will directly pass through the control circuit, and it is necessary to separate the two at this time.
Step-down switching power supply PCB upper layer diagram (the lower layer is the ground layer) Switching power supply PCB layout summary
8 main points:
1. The capacitance of the bypass ceramic capacitor should not be too large, and its parasitic series inductance should be as small as possible. Multiple capacitors in parallel can improve the impedance characteristics of the capacitor; 2. The parasitic parallel capacitance of the inductor should be as small as possible, the inductor pin pad The longer the distance, the better; 3. Avoid placing any power or signal traces on the ground;
4. The area of the high-frequency loop should be as small as possible;
5. Via placement should not destroy the path of high-frequency current on the ground; 6. Different circuits on the system board need different ground layers, and the ground layers of different circuits are connected to the power ground layer through a single point; 7. Control chip to the upper end The loop of the driving circuit with the lower-end FET should be as short as possible; 8. The switching power supply power circuit and the control signal circuit components need to be connected to different ground planes. These two ground planes are generally connected through a single point.