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So, I was revisiting some Jim Williams design notes and I came across this interesting circuit for a 5V to 200V DC converter. I though this could be an interesting platform to inspire small tube preamps from a USB power supply or battery. But, then I though there is no good isolation from a high voltage fault. So, how would you go about mitigating the risk of exposure in the event of a fault? I suppose you could use a ADC/DAC at the input, but would latency be a problem? Maybe an RF connection to decouple the physical connection? In any case, here is the circuit with an excerpt:
"Avalanche photodiodes (APD) require high volt-age bias. Figure 39.48’s design provides 200V from a 5V input. The circuit is a basic inductor flyback boost regulator with a major important deviation. Q1, a high voltage device, has been interposed between the LT1172 switch-ing regulator and the inductor. This permits the regulator to control Q1’s high voltage switching without undergo-ing high voltage stress. Q1, operating as a “cascode” with the LT1172’s internal switch, withstands L1’s high volt-age flyback events18. Diodes associated with Q1’s source
terminal clamp L1 orginated spikes arriving via Q1’s junction capacitance. The high voltage is rectified and filtered, forming the circuit’s output. Feedback to the regulator stabilizes the loop and the RC at the VC pin provides frequency compensation. The 100k path from the output divider bootstraps Q1’s gate drive to about 10V, ensuring saturation. The output connected 300˜-diode combination provides short-circuit protection by shutting down the LT1172 if the output is accidentally grounded. The 200k trim resistor sets the 200V output ±2% while using standard values in the feedback divider.
Figure 39.49 shows operating waveforms. Traces A and C are LT1172 switch current and voltage, respectively. Q1’s drain is trace B. Current ramp termination results in a high voltage flyback event at Q1’s drain. A safety attenuated version of the flyback appears at the LT1172 switch. The sinosoidal signature, due to inductor ring-off between conduction cycles, is harmless."
"Avalanche photodiodes (APD) require high volt-age bias. Figure 39.48’s design provides 200V from a 5V input. The circuit is a basic inductor flyback boost regulator with a major important deviation. Q1, a high voltage device, has been interposed between the LT1172 switch-ing regulator and the inductor. This permits the regulator to control Q1’s high voltage switching without undergo-ing high voltage stress. Q1, operating as a “cascode” with the LT1172’s internal switch, withstands L1’s high volt-age flyback events18. Diodes associated with Q1’s source
terminal clamp L1 orginated spikes arriving via Q1’s junction capacitance. The high voltage is rectified and filtered, forming the circuit’s output. Feedback to the regulator stabilizes the loop and the RC at the VC pin provides frequency compensation. The 100k path from the output divider bootstraps Q1’s gate drive to about 10V, ensuring saturation. The output connected 300˜-diode combination provides short-circuit protection by shutting down the LT1172 if the output is accidentally grounded. The 200k trim resistor sets the 200V output ±2% while using standard values in the feedback divider.
Figure 39.49 shows operating waveforms. Traces A and C are LT1172 switch current and voltage, respectively. Q1’s drain is trace B. Current ramp termination results in a high voltage flyback event at Q1’s drain. A safety attenuated version of the flyback appears at the LT1172 switch. The sinosoidal signature, due to inductor ring-off between conduction cycles, is harmless."
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