IEEE Consultants' Network of Long Island

Two Pole DC Rail Voltage Decoupler - John Dunn, Consultant, Ambertec, P.E., P.C.

The following sketch is of a two-pole rail voltage decoupling circuit.  

 
The circuit works by doing a current division between capacitor C1 and the remainder of the circuit whose impedance is examined as follows:   

Transformers With Tertiary Windings - John Dunn, Consultant, Ambertec, P.E., P.C.

We were trying to make 100 kV power supplies using the push-pull class-C amplifier technique and we got into trouble with our high voltage transformers.

With a x20 voltage multiplier on the transformer's secondary side, reflected capacitance into the primary winding was very large. Even with using 0.003 inch gapping on our EC70 transformer cores, the resonance between the primary winding's magnetization inductance and that very large reflected capacitance was at far too low a frequency.

TLV431 Oscillation - John Dunn, Consultant, Ambertec, P.E., P.C.

Texas Instruments offers the TLV431 series of shunt voltage regulators. Someone had made use of this particular device in a circuit design and just to keep things nice and quiet, he had provided the chip with a 0.1 µF bypass capacitor.

Wrong move!

That value of 0.1 µF is right smack dab in the middle of the highlighted sketch below of capacitance values that can render that regulator unstable and oscillatory and it did.   

Rail Voltage Response to Current Pulses - John Dunn, Consultant, Ambertec, P.E., P.C.

Rail voltage responses to current pulses are analyzed using recursive differential equations and two different current pulse models on the worst case assumption that passive decoupling effects of bypass capacitors are all that we have working in our favor..

It is shown in this particular case that rail voltage excursions in response to current pulses of 3 nSec rise and fall times do not exceed 40 mV per Ampere of peak pulse current.

The recursive differential equations are as follows:

Turn-On of AC Voltage - John Dunn, Consultant, Ambertec, P.E., P.C.

You might think intuitively that by switching AC power into any load at the zero crossing of the line voltage's sine wave, that you would minimize input current surges, but that conception is not always true.

If your load involves a power transformer with which you will have a magnetization inductance, you will be better off to close the on-switch at the peak of the AC voltage waveform at its 90° (or 270°) time.

This point is illustrated in the following sketch which is derived using the recursive differential equations method.   

Lightning Transients - John Dunn, Consultant, Ambertec, P.E., P.C.

Document RTCA/DO-160 Section 22 has some voltage and current waveforms to represent lightning induced transient events. Those waveforms are sketched below and descriptive algebraic equations of each waveform are shown which may be useful in transient protection analyses.

 

Text can't be copied from the above image so the text below is provided for that purpose:

Choosing Resistors For Transient Absorbing Diodes - John Dunn, Consultant, Ambertec, P.E., P.C.

Many military aircraft provide on board power via a +28 VDC bus whose actual voltage is permitted to deviate from its nominal value according to certain specified limits, including voltage transients. Those transients can be quite destructive of electronic equipment.

Protection of electronic equipment can often be provided by transient absorbing diodes, but such diodes cannot simply be connected across the +28 VDC bus. They require some kind of current-limiting series impedance between themselves and that line, usually a resistance of some value, R. Without current limiting, the diode itself will almost certainly be destroyed when the transients occur. See Figure 1.

 

An Air Pocket - John Dunn, Consultant, Ambertec, P.E., P.C.

You have this high voltage assembly and you're really proud of it. The output is 50 kV (50,000 volts) and you've provided it with triple that voltage rating worth of high voltage dielectric material. Your insulating material is thick enough to be rated for 150 kV. That three-to-one safety factor should be just fine.

The assembly runs for a month or so and suddenly there is a total internal failure. The 50 kV of high voltage has managed to penetrate and break down the 150 kV worth of insulation.

Now you ask: "Why??????"

Rise Times of Voltage Multipliers - John Dunn, Consultant, Ambertec, P.E., P.C.

A voltage multiplier does not achieve its full output voltage instantaneously upon the application of its AC input voltage. The half-wave doubler shown below and analyzed by the recursive differential equation method, illustrates the point.

The ratio of the two capacitors of this circuit, the ratio of C1 to C2 affects the DC output rise time, but there is always some rise time slowness. 

Voltage Multiplier Rise Time - John Dunn, Consultant, Ambertec, P.E., P.C.

A voltage multiplier does not achieve its full output voltage instantaneously upon the application of its AC input voltage. The half-wave doubler shown below and analyzed by the recursive differential equation loop, illustrates the point.

The ratio of the two capacitors of this circuit, the ratio of C1 to C2, affects the DC output's rise time, but there is always some rise time slowness.