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High voltage regulation, what makes a good HV regulator?
(Warning, tech talk follows!)
During the past few years, voltage regulator design has improved by leaps
and bounds for low voltage regulators. Highly efficient and small sized
switching regulators have almost completely replaced the old workhorse
LM323K and other old linear type regulators.
But, due to lack of demand, the design of higher voltage regulators
(60 to 200 volt range) has remained constant and relatively unchanged.
Overly Simplified Definitions:
Linear voltage regulator -- a voltage regulator that uses a large transistor as if it was a variable resistor. This 'variable resistor' causes a voltage drop from the incoming voltage while maintaining a sustained output voltage and current load. For a linear regulator - input current is slightly higher than output current. For example - if output voltage is 5 volts at 3 amps then output power = 15 watts. Output current is nearly identical to input current so input current = 3 amps. If the input voltage is 12 volts then input power is 3 amps x 12 volts = 36 watts. Notice there is a (36 - 15) or 21 watt difference between input and output power. This dropped power has to go 'somewhere' so it is converted to heat and this heat is dissipated thru large heat sinks. (For you techies, yes - this is an extremely simplified definition)
There are several types of linear voltage regulators. The simplest design is a series pass transistor with the voltage established by a zener diode (see Gottlieb System 1 or 80 high voltage regulators). Modern linear designs use the same basic design but are all included in one package (i.e. LM323K, LM7805T, TL783 or LM317AHVT).
Regardless of type or age of linear regulator - they all dissipate the dropped voltage the same way --> heat.
Switching voltage regulator -- a voltage regulator that uses switching technology to turn on and off the incoming voltage to a power storage circuit (inductor/diode and capacitor). The amount of time that the switch is turned on or off determines the final output voltage. With a switching regulator - a 'power conversion' is performed rather than just a voltage drop.
With a power conversion, the output power (output voltage x output current) is nearly identical to the input power (input voltage x input current) resulting in a very efficient design. For example - if voltage regulator is providing 5 volts at 3 amps then output power = 15W. Since output power is nearly identical to input power then input power = 15W. If the input voltage is 12 volts then input current is 15W / 12V = 1.25 amps. Note - input current is actually less than the output current!
Due to the high efficiency of the switching regulator, there is nearly no heat dissipated. This would be for a regulator running at 100% efficiency. But in real life, no regulators run at 100%, most run at 85% to 95% efficiency.
Efficiency of switching regulators vary widely and is affected by component choice. To determine the design with the maximum efficiency (least heat), examine the design of the regulator circuit:
Is the regulator type an old bipolar transistor or newer MOSFET design?
What type of output inductor does it use (torroid type inductors tend to work better than bobbin or bar style)?
Simple output catch diode or high efficiency type?
What type of output capacitors does it use? Inexpensive (high ESR) or more expensive caps designed for switching supplies (low ESR)? Look at the output caps - 85C rated caps are never the highly efficient low-ESR type.
All of these items affect the efficiency of a switching power supply. The higher the efficiency, the cooler it runs. And, the lower the efficiency, the hotter it runs. Excessive heat is the enemy of all electronics.
High Voltage Regulation
So, now that switchers have replaced low voltage supplies, what about the high voltage supplies?
Gottlieb System 1 and System 80 used a TIP31C (equivalent) based series pass transistor type regulator for it's 60VDC voltage.
Bally and Stern used a 2N3584 based series pass transistor type regulator for it's 180VDC voltage.
Likewise - Williams, Data East and others also used series pass transistor type regulators for their high voltage generation.
There is nothing wrong with series pass transistor type regulation as long as:
1) parts are sized for worst case conditions and 2) overload / short circuit protection is provided. Some boards did a good job with #2 (Gottlieb) but were very lax on #1. Parts worked well under ideal conditions but were too frail under heavier loading. Some boards had no overload protection AND were lax on sizing parts for worst case conditions (i.e. early Williams boards).
Advance the clock a few decades and now many of these old power supply boards are being replaced by "New and improved" versions of the power supplies.
For the most part, the low voltage (+5 volt) regulation sections are all 'cookbook' type designs using modern, ultra-efficient switching type voltage regulators as described above. High voltage regulators is where the major differences are present. Some boards use the newer linear voltage regulators, others use traditional, time proven designs.
My version of high voltage regulator is based on the traditional series pass transistor type regulator but with beefed up components. I chose this type of design because the parts are readily available and should be around for decades. The components on my HV regulator run cooler than the original linear supplies due to better heat sinking and are able to withstand a higher load current than the originals. Add to this design, a current fold back circuit to turn down the regulator output incase of short circuit and you have a very reliable, time proven power supply.
Other board and module designers chose to use the newer and cheaper one-piece linear regulators such as the Texas Instruments TL783 or the National Semiconductor LM317AHVT. The makers of these boards tout that they are advanced versions of linear supplies. Advanced? Sure in the fact that they cost less and take less board space. Yet, if you were to study the part internally, you will discover that internally it is actually the good, old series pass regulator with built in biasing components. It is the exact same thing as the old series pass regulators but smaller in size! Since they are linear regulators, they will be dissipating the exact same amount of heat as the old linear regulator. Unless the maker uses a better heat sink (which I have yet to see) then these parts will be running just as hot or even hotter than the originals. While some may claim that using these new regulators is a -good- thing, I disagree. Although these parts are cheaper (the real reason these were used) and smaller than a traditional series pass regulator, these new regulators suffer from one major disadvantage. They are often used in supplies with inputs in excess of 60VDC (such as the Bally solenoid driver board at more than 200VDC). BUT, these parts are designed for an absolute maximum input to output voltage differential of about 60-100 volts (LM317HV = 60, TL783 = 100V). This means the unregulated input voltage can NEVER exceed 60 volts more than the output. In theory, you would not expect this to happen BUT in real life...
Consider these very realistic scenarios:
1 -- At the instant of turn on time, Vin of a Bally HV regulator is at roughly 230VDC and at the same instant, Vout of the same regulator is at zero volts (still several milliseconds before turnon time). During this instant, there is a 230VDC differential between input and output. This is one of the two conditions that the manufacturers of these regulators repeatedly warn against - yet, some designers continue to ignore their warnings! These instantaneous differentials WILL shorten the lifespan of the HV regulator.
2 -- When there is an accidental short circuit there will be a prolonged period of time when the Vout to Vin differential is much higher than the maximum allowed by the manufacturer. A dead short tends to bring an immediate and violent death to their regulator circuitry!
Due to two of the manufacturers using this sort of regulator, I often get requests from their customers asking for replacement components. Sadly, I must stock repair parts for high voltage regulators that are failing in as little as a few weeks time.
Moral of the story - always evaluate the power supply you are purchasing. Cheaper is not always better - there is a reason they are cheaper! A little research and purchasing a better design upfront can save lots of cash, time and headaches in the long run.
