Design Spotlight: A High Voltage Soft Start Circuit for the Masses

High-voltage startup problems can quietly kill performance and hardware. This week’s Design Spotlight dives into a ±1kV soft-start circuit that solved a real-world failure in electron microscope HV amplifiers.

“A HIGH VOLTAGE SOFT START CIRCUIT FOR THE MASSES

“There are times where a system design doesn’t work when the anticipated design conclusion is missing a critical element. This happened when an electron microscope’s High Voltage amplifiers were turned on and capacitive input surge currents caused their power supplies to fold back and quit till reset. The first solution attempt was a simple R-C delayed gate voltage applied to a series HV MOSFET, which self-destructed as they went out of the safe operating VxI product range. So this circuit was contrived to solve this issue by employing a discrete transistor VxI multiplier to control the MOSFET turn-on power limit curve. Initial voltage drop across the MOS pair is sensed and applied to a differential current sensing bipolar pair controlling FET turn-on with a second bipolar pair forming a multiplying transconductance amp. This amp drives the MOSFET gate where device dissipation is then limited to a value of 100 watts when current is multiplied against the FET voltage drop for control. D6 provides a latching current to force this amp to provide full gate voltage at the turn-on cycle end. Though conceptionally simple this type of circuit required tight matching of bipolar parts used to form a predictable multiplying function, and, of course the LS312 and LS352 worked perfectly for the job. Since the HV amplifier loads had bipolar high voltages, negative and positive soft starts were made. The circuit is the same but all semiconductor polarities are reversed. HV FETs were cascoded because 1KV FETs weren’t available then; later converted to a single HV FET. Included is a plot showing input turn-on voltage, load voltage, and the load charging current. Pictured is the final board having plus and minus 1 Kv soft start circuits. This is one more example of a circuit favoring the application of precision discrete bipolar parts over an integrated solution. Circa 1998. - Kirkwood Rough”

This kind of controlled VxI limiting approach is still highly relevant today in high-voltage and sensitive analog systems.

Can you see which side is negative and which side is positive? It's staring at you.

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Figure 1: High Voltage Soft Start Circuit

Figure 2: Response Plo

Figure 3: Prototype High Voltage Soft Start Circuit Board