dual power supply problem
Some customers once encountered such an application scenario in the application process of the power module , as shown in Figure 1 below. The customer uses two power sources to supply power to the back-end circuit, and requires switching the input power supply without interruption of power supply. During this process, it is found that the back-end circuit will be damaged. After analyzing the waveforms of each node, it is found that when the DC -DC module is powered on, a peak voltage of 13.12V will be generated at the output of the module. When the peak voltage exceeds the withstand voltage of the back-end circuit of 13V, the back-end circuit will be damaged.
Figure 1 Customer's application circuit diagram
Analysis of dual power supply problems
We have learned about the dual power supply switching of the customer's board. During use, the customer first supplies power through the TYPEC interface , and then powers on the DC-DC module to establish a stable 12V power supply, and then switches to the DC-DC module for power supply. After analyzing the customer's circuit, it is found that when the power is supplied through the adapter , the 12V voltage of the adapter will be connected to the output terminal of the DC-DC module at the same time. The reason for this situation may be that the 12V voltage of the adapter is applied to the output terminal of the DC-DC module, which destroys the feedback loop signal of the DC-DC module, resulting in a spike in the output voltage of the DC-DC module, thereby damaging the back-end circuit. By comparing and verifying the starting waveform of the DC-DC module with or without the adapter power supply, it is verified that our analysis is correct. The comparison waveforms are shown in Figure 2 and Figure 3 below.
Figure 2 The output voltage waveform after the adapter is connected to the power supply
Figure 3 The output voltage waveform of direct start-up
Solution to dual power supply problems
This problem can be solved from the source or the way of transmission: from the source, it is necessary to isolate the influence of the adapter voltage on the DC-DC module; to solve it from the way of transmission, it is necessary to eliminate the peak voltage generated by the DC-DC module.
Add a diode at the output of the DC-DC module to isolate the influence of the adapter voltage, as shown in Figure 4;
Add a Zener diode in front of the back-end load circuit to eliminate voltage spikes beyond the tolerance range, as shown in Figure 5.
Figure 4 Adding a diode circuit
Figure 5 Adding a regulator tube circuit
Dual Power and Problem Solving Validation Results
The verification results of the above two schemes are shown in Figure 6 and Figure 7. Adding a diode to the output end of the DC-DC module can completely isolate the influence of the adapter voltage on the module loop, and the DC-DC module does not generate a peak voltage at all when it is powered on. However, due to the voltage drop problem of the diode, the voltage of the DC-DC module to the back-end circuit drops. Adding a Zener diode before the back-end circuit to eliminate the peak voltage generated can be seen to be effective. The peak voltage is suppressed to 12.4V, which is already smaller than the maximum withstand voltage of the back-end. This circuit has obvious effects. If the voltage range requirements of the back-end circuit are not particularly strict, adding a diode at the output of the DC-DC module can solve the problem more quickly; if the voltage requirements are strict, it is more suitable to connect a voltage regulator before the back-end circuit.
Figure 6 Adding the diode starter output voltage waveform
Figure 7 Increase the output voltage waveform of the Zener tube start-up