Tuesday, October 29, 2013

Yes, Virginia, CRT power supplies are still made in the USA!


CRT Displays 

Back in the day, the only kind of monitor available was a CRT type, be it computer, T.V., cockpit displays, oscilloscopes, etc. They were big, bulky and weighed a ton! And they used a lot of power.

If I left my small, color T.V. on in the morning, I’d come home from school later that day to find the temperature in my room about five degrees warmer than the rest of the house (I kept the door closed because my mom hated looking at the mess in my room. She could have made me clean it up, but she picked her battles wisely, and that wasn’t one of them. A story for a different day perhaps….). 

CRT Displays and CRT Power Supplies 


Fifteen or twenty years ago, VMI used to sell a lot of high voltage multipliers to a well-known maker of oscilloscopes. The ‘scopes were analog back then. The company had their own version of CRT power supplies, and we supplied the high voltage section. 
CRS Series - High Voltage CRT Power Supplies

From the outside, the high voltage sections looked like a big, black brick with an anode connector on one end, and flying leads on the other. They weighed as much as a brick too. 

Most CRTs and CRT power supplies have a couple of things in common. In a high voltage module, there can be various voltage or current taps, feedback networks, and such, but they all have an anode, a cathode, and one or more electrodes. 

Anodes are used to accelerate electrons, cathodes are used as the source of electrons, and electrodes are used to focus the electrons. The stream of electrons form an electron beam, which is directed to a luminescent screen. The beam is directed either by electric or magnetic deflection, depending on the application and size of the monitor. 

All in all, it’s pretty complicated, but well-established. The whole process happens extremely fast, and repeats continuously.
Image Credit:  Mediahex

LCD and CRT Displays 

These days, LCD screens are more common. They are less expensive, less bulky, and lightweight. They are not without trade-offs though. They do not have the color fidelity that CRT type displays have, and they are not as responsive. 

In comparison to CRT types, they are limited in the number of colors available. CRTs are great for high-end applications needing a wide range of colors, deep blacks, accurate color rendition, speed, and fidelity. For most CRTs, you need a CRT power supply. 

CRS Series of High Voltage CRT Power Supplies 

Anyway…..when it comes to CRT type power supplies, VMI makes the best. Our CRS series is adjustable up to 18kV (anode voltage), and ranges from zero to 550uA (anode output current). 

The CRS series features very low ripple (no noise or fuzziness in your display), excellent regulation (no droopy signals), and is arc & short circuit protected. 

Two RoHS compliant models are available, and, as always, guaranteed. The standard power supplies feature an insulated high voltage leads.

If you need a custom lead, send us your spec. We may be able to customize your power supply. VMI is ISO9001:2008 certified. 


Thursday, October 24, 2013

Single Phase Bridge Thermal Paths

Single-phase bridges are hot!  Especially high voltage ones.  Bordering on power modules, and highly application specific, high voltage (1kV and higher) 1P bridges are the thermal divas of the rectifier world.  Why is that?  Well, partly due to the magnitude of high reverse voltages, high leakage currents, and forward conduction losses.

During the recovery phase of a diode, it is blocking high voltage.  In a less-than-ideal world, the high reverse voltage generates a ‘leakage’ current in the diode.  This happens in low voltage systems too, but because the reverse voltage is much lower, it is generally ignored.  Ignore it in high voltage systems at your own risk!  Here’s why….

Peak Reverse Power

Peak reverse power is defined as the product of peak reverse voltage and leakage current, and is expressed as


P(reverse) = Vrwm * Ir

At Vrwm = 10kV, Ir = 500nA, P(reverse) = 5mW.  No a big deal, right? 


Forward Dissipated Power

Okay, so now let’s add in the forward power expressed as 


P(forward) = Vf * If

Where Vf = Forward Voltage drop, and If = Forward Current.  If Vf = 8V, and If = 1A, then P(forward) = 8W.


Total Dissipated Power

Total power, Ptot, is the sum of P(reverse) and P(forward) and is expressed as  

P(tot) = P(reverse) + P(forward)

In this example, Ptot is mostly Vf * If, and is slightly more than 8W (8.005W to be exact).  That’s at room temp (25°C).  But what if the base plate temperature is 50°C, and the thermal impedance of the device is 3°C/Watt?  That means the diode junction temperature is approximately

T(diode junction) = T(base plate) + T(Thermal Imp) °C/W * W

Or, T(diode junction) = 50°C + 3C/Watt * 8 Watt = 74°C.


The Impact of Reverse Recovery Time and Thermal Runaway

Did you know that for every 25-degree rise in temperature, leakage current triples?  It’s partly explained by temperature-sensitive reverse recovery times.  At temperatures go up, reverse recovery time gets longer which means the time to dissipate reverse power increases too.   At a diode junction temp of 74°C, the leakage current will be approximately six times the leakage measured at room temp.  In our example, at T(base plate) = 50°C, leakage current of the diode will be approx. 6 * 500nA = 3uA.  Now reverse power becomes 30mW, compared to 5mW at 25°C. 

As the temperature goes up, so does reverse power, which means the diode junction gets hotter, which means the reverse recovery time gets longer resulting in even more reverse power dissipation.  It’s easy to see how and why thermal runaway happens. 

The good news is thermal runaway can be prevented using one or more techniques.  Keep operating temperatures as low as possible, add heat-sinking capabilities, and derate components as much as as you can.  

Of course, your numbers will vary from this example.  There are other mitigating factors such as frequency and duty cycle, input signal, and the absence or presence of heat sinks.

Don't hesitate to call us if you have any questions about our single phase bridges or diodes.  We're here to help.

Wednesday, October 9, 2013

"Stacked" Single Phase Bridge Rectifier Assemblies

Have you heard the phrase, “stacked single-phase bridges”?  Its industry slang denoting an assembly of two or more single-phase bridges internally connected to one another.  That is, usually connected to one another, but not always.  I’ve also heard it used in reference to multiple single phase bridges that were not internally connected.  It’s a small difference but effects how a device can be used.


A typical, simplified schematic looks like this –   
Typical Schematic of Three "Stacked" Single-Phase Bridges 


When single-phase bridges are stacked and share an internal connection such as the D7/D8/D9/D10 node, external capacitors can be connected between their terminals creating a high voltage, high power, full-wave multiplier. 
These multipliers are usually quite large and can handle larger than average output currents.  The stacked bridges can look something like this –
Encapsulated Stacked Bridges Illustrating Terminals and Heat Sink

Alternatively, like this!
Just Kidding!
 The device above (no, not the Sponge Bob Square Pants version, the one above that) shows multiple terminations and an “L” shaped base plate. 

Advantages and Disadvantages

When an assembly has multiple bridges that are not internally connected, it’s still possible to use it as a multiplier, but the D.C. output from the first bridge must be externally connected to the D.C. input to the next bridge. 

Multiple bridges with an external connection have the added advantage that each diode anode/cathode can be accessed from the outside.  Sometimes this is convenient when monitoring signals or diagnosing problems or failures. 

Single-phase bridge assemblies vary from simple to complex.  The application and operation environment influences the choice of schematics and options.

Customizable Features

Customizable features include voltage, current, reverse recovery time, and other electrical characteristics.  Terminations, heat-sinking strategies, encapsulation, mounting methods, thermal conductivity are all specifications that can be customized, depending on your needs.  Custom housings can be developed too.  Housing materials can range from metal to epoxy, to Ultem and other thermo-plastic materials. 

VMI supplies standard bridges up to 45kV/leg, and we design custom bridge assemblies.  Just send us your specifications.