high voltage driver
| > gnd >R1 > | |B --- GND T1:NPN Power Transistor:2N3055 C/ \E | / \ | | TVS | | ----+->|<--+----+
TVS: This is a back EMF snubber. It may composed of a variety of components to suit. Sometimes you see zeners, or RC circuits, or transorbs (transient voltage suppression diode). Transil diodes provide high overvoltage protection by clamping action -- Part number 1.5KE15CA
A neon indicator bulb with a resistor in place of the TVS can be used to indicate when EMF is over 100 V.
To dissipate inductive voltage spikes across the switching gate (MOSFET or IGBT) use a fast switching diode such as a 1n5404. For low frequency pulsed circuits an ordinary 1N4001 through 1N4007 would suffice.
Designing RC Snubber Networks
L = N2 * p * A / l
- L = inductance in Henrys
- N = number of turns of wire (straight wire = 1)
- p = permeability of the core material
- A = are of the coil in square meters
- l = average length of coil in meters.
Determine saturation by noting curve of values at different power levels. For example, determine when temperature sharply swings up as voltage or load increases.
Many smaller high voltage power supplies use pulsed transformers, such as ignition coils and flyback transformers. These transformers are designed to operate with pulsed input rather than a sine wave.
Pulse Signal Generators
Pulse signal source may come from almost any signal generator. Using a 555 timer (Russian K1006ВИ1) in astable mode is common. It is cheap and easily replaced. The down-side is that you cannot adjust pulse frequency and duty-cycle separately; you must adjust on-time and off-time, which makes frequency and duty cycle interdependent. A simple 2 transistor astable multivibrator also works.
Pulse Switching Gates
The pulse signal from most signal generators cannot be fed directly into the pulse transformer because a signal generator normally cannot source much power. The signal must be amplified. A signal may be fed into a simple gate -- a bipolar transistor, or a MOSFET, or an IGBT are common gates used.
Gate devices are pretty simple to drive, but there are dedicated gate driver circuits used as interfaces between the signal source and the gate. These typically are used to convert the signal to the voltage/current levels required by the gate; isolate the signal generator from harmful feedback from the gate (transient voltage suppression -- inductive spikes); provide feedback on current and voltage moving through the gate; and provide over-current and thermal shutdown protection. ... Some gate devices (especially IGBTs) often include some TVS protection features built into the device. These usually include freewheeling diodes. Warning: be sure to understand the manufacturer's intent for these built-in protection features. Do they protect the gate and the signal generator, or are they intended to protect only the gate?
Bipolar transistors: 2N3055 -- these are inferior for driving high voltage transformers. You seem them used quite often because they are cheap, common, and have been around forever.
MOSFET: IRF740, IRFP450, IRFP460
IGBTs: 25N120, 30N120
Measuring High Voltage
Potential Transformers or Voltage Transformers are suited only for narrow applications. They are designed to measure circuits that can source a lot of current and operate at a fixed AC frequency and waveform, such as 60 Hz sine wave. (Note that these devices can often be run in reverse as good high voltage transformers).
A general purpose DC high voltage probe will typically be made from a resistor voltage divider. Many high voltage sources can drive only very low currents. Therefor the voltage divider must be very high impedance so as not to present a significant load. Resistances as high as 10000000000 Ω (1e10 Ω, or 10 GΩ, or 10 gigaohms) may be required.
Fixed bandwidth AC High Voltage probes are made with a capacitive divider network.
Note that High Voltage High Frequency probes have different requirements than High Voltage Low Frequency probes. Remember, High Frequency does not imply AC. A pulsed DC voltage source may be High Frequency.
- Basics of High Voltage Probe Design
- Bob's High Voltage Probe
- Heathkit high voltage probe uses a 1090 MΩ resistor. The 1090 megohm resistor brings the total input impedance of a Heathkit VTVM up to 1100 megohms. This yields a 100:1 scale of DC voltage measurements.