A versatile CCS load

I’m a heavy user of CCS loads. I generally use them to test my valves regardless of using my curve tracer or not. I tried multiple CCS types in the past with good results until I ended up burning one FET or protection zener or whatever due to the abuse of it.

Testing high current loads is not easy at high voltages. The DN2540 is rated at 400V. Not enough. You can use an expensive 01N100D which is another depletion 1KV MOSFET that has a lower Ciss (54pF against 200pF) or you can look at the cheaper enhancement FETs which require a different bias arrangement. If we are looking at modifying the classic cascode self-bias pair, it is a convenient opportunity to improve the VDS bias of the lower FET to improve the frequency response by lowering the Ciss. Remember that in a FET the Ciss is proportional to the VDS. The classic cascode pair has a disadvantage as the lower FET is biased with VDS lower than 1-2V to ensure the upper FET is biased correctly.

I picked up a STP3NK60ZFP which I had at hand and is a nice 600V part, includes protection zeners and Ciss is about 300pF. Not great for high-frequency response compared to depletion mode FETs, but good enough for my general use. A plastic TO-220 version can handle 20W, and more than double can be achieved with a metal TO-220. I use the CCS load for very short periods of time, so plastic is fine and simplifies the mechanical mounting of this device.

The disadvantage of using the enhancement FET is that bias is a tad more complex. I use a simple arrangement of a stable 1mA CCS formed by a pair of LND150 FETs in cascode feeding a pair of stacked 15V zeners like shown below:

The top zener (Z1) provides the right VDS for M4. M1 and M2 forms the 1mA CCS which current is set by R3. You will have to AOT R3. Z2 instead provides the bias for lower FET. This FET turns on about 3.75-4V but to use a wider range of potentiometers to set the CCS current, I elevated the zener voltage to 15V to allow a pair of 500Ω and 5KΩ multi-turn potentiometers to be used for coarse and fine adjustment. I actually had them at hand from my previous CCS load.

The CCS load will work with at least 35-40V across it and will deliver currents from 2.5mA to 200mA. Higher currents can be achieved, however I added R6 to protect the potentiometers and FETs from an unexpected short at high current for a period long enough which will guarantee the death of the CCS. The LND150 are rated at 500V so probably it would be safer not to operate this CCS at voltages higher than 530V.

To illustrate the improvement of the FET Ciss, we just need to look at the datasheet to see that with just 15V we managed to reduce the FET’s capacitances to minimum levels.

If you add a 100uF capacitor between cathode of top zener (Z1) and source of bottom FET (M4) you can improve the frequency response and also provide a soft start to the CCS. This cap needs to be rated at 35V at least. I haven’t tried this yet and seems quite useful for a fixed CCS use.

Looking at SPICE simulation we can see how well it performs:

The attenuation at 100Hz is above 100dB and at 20kHz is good enough with 70dB.

I built it in a couple of hours using a remaining piece of PCB:

With this CCS you can do multiple tests. In fact if you don’t have a valve tester and you do have an oscilloscope and generator you can measure transconductance, mu, match pairs, identify old valves, etc.  You can even use this load to measure inductance of transformers and chokes with a bridge.

Author: Ale Moglia

"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp) View all posts by Ale Moglia