Common failures of a laser power supply

Its to be expected that eventually a part will fail, but what about DC excited laser power supplies?
Why is the failure of this component inevitable? Is it due to cheap construction, or is there more to this?

In this article we will explore the potential failure modes of a laser power supply.

First we must understand what is the purpose of the power supply and how does it operate.
A dc excited glass laser tube requires a very high voltage at a moderately low current in order to sustain an electrical arc inside of a vacuum of inert gas’s 
When we say high voltage, we really mean it, we are talking in the region of 28- 50 kV  that’s 50,000V.
Obviously the mains voltage you are feeding into the machine must somehow be converted to this obscene voltage.
This happens in a number of stages.

High Voltage Safety

  • Work on un-energized circuits if at all possible.
  • Be very careful around live 50/60 Hz electricity, since it requires very little current to injure. Your power supply can kill you!
  • Limit the current and energy to the lowest values possible. Lots of interesting experimentation can be done with low stored electrical energy and low currents of a few mA or even microamperes. Make it a habit to ask yourself if you really need this current or energy.
  • Keep your distance from live high voltage circuits. Since high voltages can breakdown air to connect you to a circuit, keep high voltage circuits in enclosures and behind barricades when in operation.
  • Be sure to properly ground your experiment and your enclosure. Take special care to safely de-energize and ground a circuit before working on it. Know when and how you can end up in the ground path in a circuit and put safeguards in place to eliminate this eventuality.
  • Never work alone, always have a partner who knows your equipment and the risks and hazards involved. That way, you have a second set of eyes to insure safety, and someone who can shut off the power and get help if you are injured.

Disclaimer

The authors do not make any representations as to the completeness or the accuracy of the information contained herein, and disclaim any liability for damages or injuries, whether caused by or arising from the lack of completeness, inaccuracies of the information, misinterpretations of the directions, misapplication of the circuits and information or otherwise. The authors, contributors, and owners expressly disclaim any implied warranties and of fitness of use for any particular purpose, even if a particular purpose is indicated in the text.

Stage 1 – rectification

The AC voltage must be converted into a DC voltage, this happens using a bridge rectifier circuit, the converts the sine wave of an AC signal into positive voltage, it does this by inverting the negative part of the sine wave using a number of diodes in a wheatstone bridge configuration.

Stage 2 – conditioning

The rectified voltage although dc is far to lumpy to be considered suitable for use in this application, the signal must undergo some treatment to smooth the highs and low into an acceptable signal. The rectified voltage is smoothed by 2 large 820uF capacitors.

Stage 3 – switching

The now smoothed dc voltage must now be modulated, this is how the output power is varied. The conditioned voltage from stage 2 is fed into a pair of fast switching power transistors (Mosfets), mosfets are similar in principal to a relay with some very important differences. The output of this stage is known as the rms voltage (root mean square).
The voltage is switched on and off, the time on vs off is known as the duty cycle. This duty cycle once averaged over time can be considered the average output voltage, although this is always 100% of the input voltage but modulated over time.

Stage 4- voltage transformation

The now dc voltage around 99v if 220v input or 55v is 110v input must be converted to an extremely high voltage. This happens inside of a pair of high voltage flyback transformers. The flyback transformer is a large inductor that stores energy and a primary & secondary winding.
The primary winding is connected to the dc voltage via the power mosfets whist in the on region. The inductor stores the voltage, once the mosfet disconnects the primary winding from the dc voltage the energy is transferred to the secondary winding.
The flyback transformers step up the voltage in stages.

flyback one 

input voltage 99v dc
Primary transformer turns 10
Secondary transformer turns 50
Transformer ratio 1-5
Input voltage x transformer ratio = output of secondary
99v x 5 = 495V

flyback two

input voltage 495v dc
Primary transformer turns 10
Secondary transformer turns 50
Transformer ratio 1-5
Input voltage x transformer ratio = output of secondary
495v x 5 = 2,475V

Stage 5 – Excitation

The now high voltage dc is supplied to the laser tube via the high tension cable (high voltage cable) for this stage i will not explain to much depth due to the complexity of how it happens and the physics behind why it must happen. But each time the laser is pulsed the excitation voltage must be in the magnitude of 10x operational voltage of the laser, This is known as the striking voltage. This is achieved using a multiplier circuit to boost the voltage at a ratio of 1-10.

So now we have a basic understanding of how the power supply creates the voltage required to strike the arc and sustain inside the tube, let’s take a few moments and do a quick rundown of the control system that regulates the power at the tube and how that system works.

Power control module

The power control module as its name suggests is in charge of regulating the output power of the laser tube, a signal is supplied from the laser controller named IN. This signal is pulse-width modulated and must undergo some processing before being passed onto the fast switching MOSFETs that supply the flyback transformers. The signal is decoupled from the high voltage using an optical coupler, this converts the electrical signal into light, before converting the light back into an electrical signal. This allows for the two systems to be isolated from one another. This signal must then be passed through an inverting hex Schmitt trigger, the purpose of which is to ensure the signal is crystal clear and is either high or low and never between the two.
The power control module must also ensure that the water switch is low before allowing the signal to be passed to the MOSFETS, this is done using a 2 input inverting or gate. Both inputs must be low in order for the output to be low, due to this being an inverting or gate a 0 at the output results in a logic high signal.

 

What can go wrong?

So now we have a basic understanding of how the laser PSU operates, we can begin to talk about the failure modes of this component.
We will start with the most common failure of the flyback.

Flyback transformer failure.

The flyback transformer is essentially an iron donut with some cables wrapped around it, the primary winding and secondary windings and a smaller feedback winding. Manufacturing defects, & overheating are the leading cause of failure. A number of scenarios can occur when this component fails, most commonly is reduced power at the laser tube & a screeching sound coming from inside the power supply. It is a dangerous failure as high voltage can leak to ground and cause a potential difference of a few thousand volts, if you couple this with the fact these machines notoriously have bad wirings you could receive a nasty shock.

Fast switching MOSFET failure

The switching transistor is another common failure, transistors are sensitive to high voltage electrostatic discharge and get damaged in the event of the laser tube going open circuit or the flybacks failing. Sometimes the failure may not occur at the same time as the discharge, but perhaps hours to weeks later. Mosfets can fail both open or closed, this will mean the tube either constantly fires or refuses to fire.

Bridge Rectifier failure

The bridge rectifier is usually an easy failure to spot. If the input protection fuse is ok, yet the fan refuses to spin the bridge rectifier will most probably have failed. Another component leading to the rectifier is the metal oxide varistor, the purpose of the varistor is to protect against transient voltage spikes.

Power control module failure

The power control module has multiple points of failure, with the most common being the inverting hex schmitt trigger. When this fails the laser will begin to output strange results that would make one think the controller is at fault. As the power supply begins to heat up during use, the power supply will fail to correctly regulate the power output to the tube. The laser will continue to fire at a low power as opposed to being turned off, an example of this can be seen as faint lines between cut out circles whist the head is moving to the next cut position.

What can i do in the event of a power supply failure?

Atklaser stock power supplies of all sized and wattage. from 40w to 200w for dc laser power supplies.
All our listings for power supplies include a surcharge repayable on return of your failed power supply for recycling. Check out our laser power supply exchange refund terms and conditions.

 

View our range of power supplys

£149.99

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