Graphic explanation: switching power supply test steps

1. Working principle of switching power supply

1. A switching power supply is a high-frequency switching energy conversion electronic circuit, often used as a power supply for equipment. Common conversion categories include: AC-DC, DC-DC, DC-AC, etc.

2. Switching power supply block diagram

(1) After the mains electricity enters the power supply, it first passes through the front-end EMI filter circuit. The main function of EMI filtering is to filter out the interference of high-frequency pulses from the external power grid on the power supply, and at the same time reduce the electromagnetic interference of the switching power supply itself to the outside world. In fact, it takes advantage of the characteristics of inductance and capacitance to allow AC with a frequency of about 50Hz to pass through the filter smoothly, while high-frequency interference clutter above 50Hz will be filtered out by the filter.

(2) After EMI filtering, the relatively smooth sinusoidal AC is sent to the front-stage rectifier circuit for rectification. The rectification work is performed by the full-bridge rectifier diode. After rectification by the full-bridge rectifier diode, the voltage is completely converted into a positive-phase voltage. However, the voltage obtained at this time still has large fluctuations, so it is necessary to use a high-voltage filter capacitor for preliminary voltage stabilization to correct the waveform to a waveform with smaller fluctuations.

(3) Converting DC power into high-frequency pulsating DC power is done by the control circuit. The output part is fed back to the control circuit through a certain circuit. The control circuit is used to adjust the switching time ratio of the high-frequency switching element to achieve the purpose of stabilizing the output voltage. The control circuit has been integrated and made into various integrated circuits for switching power supplies.

(4) The pulsating DC power is sent to a high-frequency switching transformer for voltage reduction. The low-voltage filter circuit composed of a diode and a filter capacitor then rectifies and filters the resulting pure low-voltage DC power for use in the equipment.

3. Switching power supply features:

(1) The switching power supply is a nonlinear power supply with light size and weight.

(2) The power transistor works in a switching state, the power consumption of the transistor is small and the conversion efficiency is high.

2. Switching power supply test method

1. Test items: loop gain, output impedance, output ripple, switching noise, etc.

2. Loop gain test:

The switching power supply circuit can be regarded as a simple feedback control system

A negative feedback loop will generate self-excitation when GH=-1 (GH is called open loop gain).

Gain). Decomposed into: Amplitude condition: |GH|=1, Phase condition: GH phase Φ=-180º
The open-loop characteristic is a very important parameter that characterizes the stability of the feedback system. It is usually expressed by gain margin and phase margin:
Gain margin: Φ=-180º, 0-Gain (dB)
Phase margin: Gain=0, Φ-(-180º)

Usually expressed as a Bode plot

When testing the open-loop characteristics, the switching power supply should work in a closed-loop state to ensure the stability of the system state. The test plan is as follows:

(1) Test equipment:

4395A, 41802A*2 (50ohm to 1Mohm adapter), 10441B*2 (probe), 9100-0855 (transformer)

(2) Test block diagram:

The output of the switching power supply is added with an electronic load; the input of the error amplifier is connected to the R port of the 4395A through the 41802A 1 MΩ input adapter and the 10441B probe; the output of the switching power supply is connected to the A port of the 4395A through the 41802A and 10441B; the RF output of the 4395A is added to the input of the error amplifier and the output of the switching power supply through a transformer. The R channel is used to measure the signal injected into the loop, and the A channel is used to measure the output signal of the loop. A/R is the open-loop characteristic of the loop. To ensure the accuracy of the measurement, a through calibration is required before the test.

(3) Notes during testing:

A. In order to effectively inject RF energy into the DUT, the impedance of the transformer cannot be significantly lower than the impedance of the 4395A RF output port (50ohm); the transformer cannot have a resonance point within the test frequency range and should be inductive to effectively inject the signal. The transformer characteristics can be tested using an impedance analyzer and a 16047E test fixture:

B. The RF output signal of 4395A should not be too large or too small. If the signal is too small, all that is measured is noise, and if it is too large, the feedback loop will be saturated. The signal size depends on the transformer, and the saturation depends on each switching power supply, so there is no fixed value.

You can change the excitation power during measurement and look at A/R, A, and R to determine an appropriate excitation power.

(4) Test result reading:

A. Usually it is measured under full load, half load and zero load conditions;

B. Phase should be read under unwrap

C. Gain margin: loop gain when the phase of A/R is 0

Phase margin: The phase value when the loop gain is 0dB

As shown in the figure above: the phase margin is 67.4º at 580Hz (line A); the gain margin is 38.1dB at 18.6KHz (line B). The reading method of the gain margin and phase margin at this time is slightly different from that introduced in the principle part. The reason is that when measuring, the A/R includes the error amplifier (not just the GH), and there is a 180-degree reverse change.

(5) Requirements for test equipment for loop gain testing:

The test equipment required is 4395A (network analyzer) with a frequency coverage range from 10Hz to 50KHz or higher frequency range from 10Hz to 500MHz

Narrow bandwidth IFBW 2Hz to 30KHz

High frequency resolution Test points: 2 to 801; Frequency resolution: 1mHz

Logarithmic scan Scan mode: Logarithmic scan

Gain and Phase Mode Format: LogMag and Phase

Marker function - minimum 4 (2 per margin) or Delta-Marker 8 Markers and Delta-Markers

Minimum excitation power output power -60dBm to 20dBm

3. Output impedance

(1) Test equipment:

4395A, 41802A*2 (50ohm to 1Mohm adapter), 10441B*2 (probe), 9100-0855 (transformer)

(2) Test block diagram:

A/R lin MAG is the test result. To ensure the accuracy of the test, a through calibration is required before the test.

(3) Issues that require attention during testing: To prevent DC power from being output to T1 from the switching power supply, place capacitor C between +out and T1. The impedance of T1 should be less than a few ohms within the test frequency range, and 470uF is recommended.

(4) Test results:

(5) Output impedance test requirements for test equipment

Requirements for test equipment 4395A (network analyzer)

Frequency range from 100Hz to 100KHz or even wider Frequency range from 10Hz to 500MHz

High frequency resolution Test points: 2 to 801; Frequency resolution: 1mHz

Logarithmic scan Scan mode: Logarithmic scan

Marker function 8 markers

Pass/Fail function Built-in limit line function for Pass/Fail testing

4. Power supply output fluctuation and switching noise test

(1) Test equipment: 4395A, 41802A (50ohm to 1Mohm adapter), 10441B (probe)

(2) Test block diagram

Use the spectrum analyzer mode to observe the characteristics of the output signal

(3) Test results

(4) Requirements for test equipment for power supply output fluctuation and switching noise testing

Requirements for test equipment 4395A (spectrum analyzer)

Wide frequency coverage Frequency range from 10Hz to 500MHz

Narrow bandwidth, fast test speed IFBW: 1Hz to 1MHz (the test speed is also fast when the IFBW is narrow)

Logarithmic scan Scan mode: Logarithmic scan

Marker function 8 markers

Pass/Fail function Built-in limit line function for Pass/Fail testing