K40 Laser Power Supply Driver Circuits

K40 Laser HV Power Driver 

As the investigation of the laser drive methods continues we now aspire to understand how HV power is applied to the K40 laser tube.
This post is a continuation of laser-power-supply-control-take-2.html, Whereas the focus in that post was the understanding and implementation of digital controls, this post continues with a focus on understanding the internal driver circuitry of the LPS.

Other related posts are:

• http://donsthings.blogspot.com/2017/04/adding-analog-milliamp-meter-to-k40.html
• http://donsthings.blogspot.com/2017/01/k40-lps-configuration-and-wiring.html
• http://donsthings.blogspot.com/2017/01/k40-lps-repair-and-test.html

Contributors:

+Don Kleinschnitz

+Paul de Groot 

+Nate Caine 

Donate:

Please consider donating (button to the right of this post).
Your donations help fund additional research, tools and parts that I will return to the community as information.

For other information on the K40-S build use the  K40-S BUILD INDEX with schematic

Warning

DON"T IGNORE THIS!

WARNING: LASERS AND THEIR HIGH VOLTAGE SUPPLIES ARE BOTH ELECTRICALLY LETHAL AND OPTICALLY DANGEROUS. THEY HAVE THE POTENTIAL TO KILL AND/OR BLIND YOU!

YOU ARE ENTERING A ZONE WHERE 20,000 Volts will be present!

1. STAY FAR AWAY FROM THE HIGH VOLTAGE SUPPLY'S OUTPUT!
2. WEAR PROTECTIVE EYE-WARE AT ALL TIMES WHEN OPERATING A K40!
3. DO NOT OPERATE A K40 WITHOUT THE PROPER LASER INHIBITING INTERLOCKS INSTALLED AND OPERATING PROPERLY!
4. USE THE CORRECT HIGH VOLTAGE SAFETY PROCEDURES

IN READING THIS POST YOU AGREE TO USE THIS INFORMATION AT YOUR OWN RISK!

I DO NOT RECOMMEND THAT YOU OPEN, PROBE, REPAIR OR OTHERWISE ACCESS THE INTERNALS OF A LASER POWER SUPPLY. YOU PROCEED FROM HERE AT YOUR OWN RISK!

Don't believe me? Here is an example of the LPS's energy!

Video by +David Cantrell

Safety

Most people are not skilled enough to be even near and certainly not inside a high voltage power supply such as this without special training and equipment. As an example, this is the specially designed and built lab environment that I use to work on Laser Power Supplies. Nothing less is acceptable. 
Hopefully, there is enough information on my blog to satiate your curiosity about your LPS or help you troubleshoot outside of the LPS's guts. 
The output of these supplies is a lethal 20,000 Volts but there are equally lethal voltages that can be found in the driver circuits (400 volts). This means that even if you disconnect the fly-back (where the highest voltage is created) lethal and high voltages are present in the voltage multiplier circuits.
These high voltages can also damage test equipment such as meters and oscilloscopes if grounding and input attenuation are not carefully planned.

LPS Schematics

The updated schematic is used as the base for the "theoretical" theory of operation given below.

The embedded schematic below .....

https://www.digikey.com/schemeit/project/k40-lps-2-EFKO7C8303M0

PWM control

Much of the PWM's operation was covered in the related post so it is not repeated here.

HSwitch

The output of the PWM drives, with a complementary signal, an Hswitch which in turn drives a transformer in a push-pull fashion. The secondary of this transformer drives the HV driver MOSFETs.

Charge/Voltage Doubler

The charge (voltage) that is dumped through the HV HVT is created using a doubler technique. Each of the doubler capacitors it charged respectively on each 1/2 cycle of the input AC through the full-wave bridge. This results in 2x the input voltage on the series combination of the two capacitors.
This reference is what helped me decode this circuit: PowerSourcesForCW-Lasers

AC Line Voltage selection

There is a selection switch on these supplies for 115 and 240 volt operation. When the switch is in the 115 volt position only one of the capacitors is charged at time so the voltage across each is equal to the line voltage (115) and when in series they add to 2x line voltage (230).

When the switch is in the 230 volt position the capacitors are charged in series so each capacitor has 1/2 AC Volts or 115 volts each. The net result with 230 VAC in is the same as 115 VAC in. 

The operation is simplified in the image below.

Voltage Doubler Operation

HV Driver

The HV driver uses the complementary signal from the Hswitch to dump the charge from the doubler capacitors through the HVT's primary. This results in a secondary high voltage that is roughly proportional to the HVT's winding ratio * primary voltage.

The image below is a simplified view of its operation. It shows that the current is dumped through the HVT in two directions, creating an AC-like signal that has a period equal to the PWM.  I need to verify this theory with a scope.

Simplified HVT Driver

Sense Transformer

In series with the HVT and in turn, its current is a transformer that converts the HVT current to a proportional DC voltage. This voltage provides current (I) feedback to the PWM controller.

Open question: This method senses current in the HVT primary but how does that know and regulate the current in the tube. The tube exhibits a negative resistance when it fires so how is the current in the tube measured and regulated, seems it isn't. 

HVT Transformer

Most K40 HVT's (in the teardowns we have done) consists of a voltage doubler.

For more info on the HVT see these posts:
https://donsthings.blogspot.com/2017/06/k40-high-voltage-transformer-autopsy-2.html
https://donsthings.blogspot.com/2017/06/k40-flyback-autopsy.html

An example HV diode  specification.

Enjoy and comment
Maker Don

K40 High Voltage Transformer Autopsy #2

Background

This is a continuation of http://donsthings.blogspot.com/2017/06/k40-flyback-autopsy.html. In the previous post both myself and +Nate Caine tore down High Voltage Transformers (HVT) as part of our quest for knowledge of the details of the K40 LPS internals.
In this tear down the potting material was removed chemically in hopes that the circuit and its components could be kept in tack. The transformer was also sectioned a means of understanding its design.

Donate:

Please consider donating (button to the right of this post).
Your donations help fund additional research, tools and parts that I will return to the community as information.

For other information on the K40-S build use the  K40-S BUILD INDEX with schematics.

The schematics for the LPS:

Removing the Potting

The potting was mostly removed using paint stripper. Caution: use gloves as this stuff is caustic.

Use gloves and eye protection this stuff is caustic

HVT submerged in glass container

The potting removal took nearly 2 weeks of repeatedly checking and refreshing the solution. The potting will come off in flakes which I washed off with each refresh. Unfortunately the stripper de-laminated the capacitors and destroyed their covering so no labels were visible. The diodes had no labels. 

It is probably worth experimenting with other chemicals that might work faster and not destroy the components coating but this worked good enough to get the information we needed.

Discovering the Circuit

The component connections were carefully observed as the potting removal proceeded. This version (and it seems there are more than one) used 2 HV diodes and 2 HV capacitors in a Voltage Doubler configuration. In this case (unlike the previous autopsy) there were no parallel diodes.

Circuit removed from the potting. Overlay showing connection of secondary.

Example circuit. These exact components are not what is used in a K40 LPS

HVT Cross Section and Analysis

After the potting removal step the transformer was sectioned with an abrasive metal blade on a Dremel and then polished on a marble flat plate with 600 grit wet paper until the wire cross sections was visible.

Left: five section secondary. Right one section primary winding's

Primary Winding

The primary winding consists of 40 turns of 21 wire bundles.

Primary winding

Secondary Winding

The secondary winding consists of 5 sections of  +400 turns wired in series.

Primary winding connection leads

One section of secondary winding

The secondary does not have a center tap and is directly connected to the junction of the diodes and the minus leg of the lower capacitor. See voltage double'r example.

HVT Design

The HVT consists of a primary winding and a 5 section set of secondary winding's.

HVT Primary

The HVT primary has 40 turns in which each turn is a twisted set of 21 wires. The primary effective wire size indicates that the primary is designed to handle much more current than the secondary.

HVT Secondary

The secondary has 5 winding sections insulated from each other but connected in series. 

The turns wound on three different secondary sections were counted, one on the first  HVT and 2 from the second HVT. The first count = 499 and the two sections counted on the second HVT = 452 and 471 respectively. 

I don't think the difference in the counts are caused by counting errors. Its seems that the turns on each section are not the same (Rt). I did not further investigate this variance in turn counts as I do not think it will materially change the outcome of the analysis.

Turns Ratio

Using an average of the last HVT 2 sections turns count lets assume:  

• Average turns per secondary section = 461.2
• Number of secondary segments = 5

Total # of turns = 461.2 * 5 = 2306 total turns on the secondary

Therefore the HVT is a 40t to 2306t HVT transformer, a ratio of 1:57.65

Estimating Output Voltage

The voltage at the output can be expressed as:

Hv = Pv * Rt * Mv

where:

• Hv is the output voltage
• Pv is the voltage on the primary
• Rt is the ratio of primary to secondary
• Mv is the voltage multiplication factor

Therefore for every 100V on the primary the output = :

Hv = Pv * Rt * Mv

11,300 = 100 * 57.65 * 2

Calculating HVT Primary Voltage

The typical K40 PS output voltage is specified at 23,000V @20ma.

If we solve the above equation for Pv:

Pv = Hv/(Rt*Mv)

And use it to estimate the primary voltage at spec: 

Pv = 23000/(57.65*2)

Pv = 200 Volts

By inspection of the LPS schematic I estimate the HV buss to run at about 240-300 volts, 40 volts larger than the estimate above..

This error of 40 volts on the primary equates to about 2,306 volts on the output or 10% of the specified output. This error can easily be the result of an error in estimating the total turns across the 5 secondary sections [perhaps each secondary section has different turns] or simply differences in any given manufactures specified HV output.

Primary Current Estimates:

The specified max current output for a typical K40 supply is 20ma. In a transformer the voltage on the output is increased by the turns ratio. So to the current in the primary is larger than the current in the secondary by that same ratio (Rt). 

Therefore: 

Pi = Si * Rt

where;

Pi = the primary current

Si = the secondary current

Rt = the turns ratio

Solving for the primary current using K40 LPS specs and given the output current = .02 amps

Pi = .02 * 57.65 = 1.1 amps

Learning's

A K40 HVT contains a high current primary and a multi-section secondary. A high turns ratio secondary in combination with a voltage double'r  creates approximately 11,300 volts per 100 volts of primary voltage.
This autopsy provides a model of the HVT that more completely characterizes a key component of a K40 LPS... its HVT.

If the above analysis holds true then the following has been verified:

• K40 LPS are easily capable of voltages in excess of 23,000 volts
• A K40 HVT's include a voltage doubl-er in its output stage
• A K40 HVT cannot be tested using a standard DVM because it cannot forward bias the internal HV diodes. Each HV diode is actually a serially connected stack of 20 or more diodes. Voltages that exceed 120 volts might be necessary to forward bias both the diodes in this double'r configuration.
• K40 HVT's are not repairable

Suspicions of K40 LPS Failure Modes

I suspect that LPS failures fall into these categories:

• AC plug swapped with the DC plug damaging the supply's enabling circuits
• The low voltage PWM controllers output shorting, blowing itself and the bridge rectifier.
• The Bridge rectifier failing under load. 
• Arc's causing excessive secondary current, opening the HVT's diodes.

What's Next To Do On the K40 LPS Quest

1. Map the actual voltages in the LPS including the primary's HV buss to further verify the above model.
2. Scope and capture dynamic views of the internal circuitry's operation.
3. Scope and capture dynamic views of the Lasers current and voltage while marking.
4. Review the component specifications and verify that specifications are not being exceeded in actual operation.
5. Noodle a safe HVT DIY HVT tester. 
6. Noodle a safe and DIY HV tester

Enjoy and comment
Don