Arc lamp power supply current / voltage regulation.

[ionics]

Greetings all.

I am needing to power some short arc lamps that operate at voltages between 20 and 55 vdc and need to be current regulated- For ex. one lamp requires 56 vdc and 3.7 amps. I was thinking of using PWM to control current / voltage. Have seen several different types of PWM such as "distributed PWM" and others. Can someone explain the difference and perhaps why one would be better than another for said application?

Any suggestions? I was going to start by having a power inductor supply the proper peak voltage and able to source the current requirements- rectify and filter.... Is it then a simple matter of adjusting the PW to a switch of some type? was thinking of IGBT's?

I only know the very basics of power supply design- if you have any input or suggestions they are very welcome.

Thank you all.

David V.

 

[Hero999]

You need an inductor as well as a PWM circuit to form a buck converter.

How are you igniting the lamp? Normally this requires a high voltage so you need to be sure the components you select can withstand the ignition voltage.

 

[ionics]

You need an inductor as well as a PWM circuit to form a buck converter.

How are you igniting the lamp? Normally this requires a high voltage so you need to be sure the components you select can withstand the ignition voltage.

I have an ignition module that will take care of the start pulse. It is a series triggering scheme with an RF component to it.

The lamps require "resonably clean" DC to operate. Most of the PWM stuff I see is related to inverter stuff or simulating a waveform.... I want low ripple D.C. so I assume I want a high freq. drive.

I am having trouble understanding the difference in regulation schemes using PWM for constant- voltage/current/power control.

I assume I need two circuits- one to adjust current and one to adjust voltage?
I am under the assumption that I would need to use the PWM circuit for the current regulation part- and to adjust the voltage, I would regulate the voltage into the switching devices?

This is confusing me, I "somewhat" understand the implications of Ohm's law and I see how
changing the duty cycle of the pulses changes the average voltage seen by the load as well as limits the power per unit time the load receives. so when I adjust the PW will I not be changing both the voltage and the current seen by the load?
How do I keep one constant while adjusting the other and visa-versa?

Any good tutorials or books to recommend?

Thanks again for the time to help

Dave V.

 

[Hero999]

I don't have any experience with constant power but constant current and voltage are pretty easy to understand.

Constant voltage, adjusts the duty cycle of the PWM, increasing if the output voltage is too low and decreasing it if it's too high. An error amplifier is used to

Constant current works on the same principle as constant voltage except that the feedback voltage is taken from a resistor in series with the load, known as a current sense resistor. The voltage across the current sense resistor is kept constant and due to Ohm's law the current through it is also constant.

Plenty of ICs can be used to regulate the voltage and current.

 

[ionics]

Thanks again for the reply, I am still a little confused. I would like to end up with a two knob style supply where I can control the voltage and current with a separate knob. I can't see how varying the duty cycle the same way via two "switches, e.g. IGBT etc..." would have any effect other than reducing the total power. If one were to draw this out in a block style diagram would one of the switches need to be controlling the input to an inductor for the voltage part and then another "switch" on the inductors output for the current part or am I just all mixed up?

 

[crutschow]

You can control the current, or you can control the voltage, but not independently do both. Ohms law doesn't allow it. But for most arc lamps the operating voltage is relatively constant and varies only slightly with current. Thus to control an arc lamps power, you need a constant-current PWM power supply. You measure the voltage across a small resistor in series with the arc lamp and use that to control the PWM duty-cycle. If you wanted to avoid the power loss through the resistor, you could use a hall-effect device to sense the current in the wire.

 

[Hero999]

Don't worry about the voltage, you only need to control the current to an arc lamp, the arc will determine the required voltage.