I've been working on a switching power supply project, and one element of the design involves over voltage and reverse voltage protection. I thought I had that part sorted out (see this thread), but now that I've built a prototype that includes the OV/RV circuit on the same board with the SMPS, I have a really bizarre problem.
When the board is powered with no load on the output, it appears to behave normally and output is 5.19V, as it should be. However, when a load is placed on the output, the two 10μF input capacitors (MLCCs) emit a surprisingly loud piezo-like buzzing sound, and output drops to ~1.4V. Buzzing frequency drops as input voltage rises, and vice versa. If the OV/RV circuit is bypassed and input power is applied directly at the MLCCs, the SMPS works normally.
I went back to an earlier SMPS-only prototype board and connected a separate OV/RV test board to it via wires. Same problem.
I've tried paralleling additional capacitance with the MLCCs. Each step up in total capacitance seems to slightly decrease the volume of buzzing, increases input current draw, and increases output voltage. I ended up with MLCCs soldered in a comical five-high tower (total 126μF), but the problem remained, and output never got above ~3.2V.
Then I went all-in and soldered a 220μF electrolytic to the MLCC tower (total 346μF). Output rose to ~5V and the buzzing was very faint, but output voltage rose and fell randomly by ~100mV and input current varied by ~250mA. Again, this SMPS is rock solid -- and dead quiet -- without the OV/RV in circuit.
I'm attaching the core schematic, bill of materials, and a short audio recording of the MLCCs in full song. The recording starts with input voltage at 7V. Vin then increases slowly to ~18V, where the OV protection transistor cuts it off, then decreases back to 7V.
Oh, and one more thing. This problem seems to be causing the OV protection transistor to overheat. Either that, or I haven't spec'ed it adequately for the board's current demands (typically 14V at 0.9-1.8A). Here are some numbers on the transistor in-circuit (Vin=14.00V):
This has me completely baffled. Does anyone have any idea what the problem could be? Let me know if any additional measurements would be useful.
Thanks.
When the board is powered with no load on the output, it appears to behave normally and output is 5.19V, as it should be. However, when a load is placed on the output, the two 10μF input capacitors (MLCCs) emit a surprisingly loud piezo-like buzzing sound, and output drops to ~1.4V. Buzzing frequency drops as input voltage rises, and vice versa. If the OV/RV circuit is bypassed and input power is applied directly at the MLCCs, the SMPS works normally.
I went back to an earlier SMPS-only prototype board and connected a separate OV/RV test board to it via wires. Same problem.
I've tried paralleling additional capacitance with the MLCCs. Each step up in total capacitance seems to slightly decrease the volume of buzzing, increases input current draw, and increases output voltage. I ended up with MLCCs soldered in a comical five-high tower (total 126μF), but the problem remained, and output never got above ~3.2V.
Then I went all-in and soldered a 220μF electrolytic to the MLCC tower (total 346μF). Output rose to ~5V and the buzzing was very faint, but output voltage rose and fell randomly by ~100mV and input current varied by ~250mA. Again, this SMPS is rock solid -- and dead quiet -- without the OV/RV in circuit.
I'm attaching the core schematic, bill of materials, and a short audio recording of the MLCCs in full song. The recording starts with input voltage at 7V. Vin then increases slowly to ~18V, where the OV protection transistor cuts it off, then decreases back to 7V.
Oh, and one more thing. This problem seems to be causing the OV protection transistor to overheat. Either that, or I haven't spec'ed it adequately for the board's current demands (typically 14V at 0.9-1.8A). Here are some numbers on the transistor in-circuit (Vin=14.00V):
- 2x 10μF MLCCs, no load: Ie=15mA, Vce=5mV
- 2x 10μF MLCCs, 1.75Ω load: Ie=180mA, Vce=410mV
- 2x 10μF MLCCs + 220μF electrolytic, 1.75Ω load: Ie=1.45A, Vce=2.5V. Smoke within ~45 sec.
This has me completely baffled. Does anyone have any idea what the problem could be? Let me know if any additional measurements would be useful.
Thanks.
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