A uTracer for higher voltages and currents with a pulsed output power up to 1kW!

Contents:
  1. Introduction
  2. Higher voltages: generation

1. Introduction

I think that I am not exaggerating when I write that the uTracer3+ has been an unexpected and enormous success. At the moment of writing these lines in excess of 1550 uTracer3+ are in service in 55 countries! The uTracer3+ is a compromise between simplicity and performance and was originally intended for my private use only. It was designed to trace and test the majority of the tubes that are being used in amplifiers, radios and televisions. Nevertheless, over the years people have continuously asked me for a tester/tracer with extended capabilities, especially towards higher voltages.

Sense and Simplicity

Personally I rather doubt the need for testing/tracing at higher anode and screen voltages. It is true that in some audio amplifiers supply voltages as high as 800 V are used, but I think that most of the interesting tube parameters can be well characterized at voltages between 0 and 400 V, while for higher voltages the curves can simply be extrapolated. Nevertheless, I agree that especially for RF transmitter tubes higher voltages would be welcome. With higher voltages comes “in one breath” the wish for higher currents. A uTracer for higher voltages and currents doesn’t make much sense if not also the grid bias range is being revised, and not only to more negative values, but for transmitter tubes also to positive grid biases. With positive grid biases also comes the need to measure the grid current …..

Figure 1.1 Studies for a new uTracer that on their way ended up in “the valley-of-death.” Left, prototype for the high voltage supply/switch of the uTracer5 (never published).
Centre, perfboard version of the uTracer5. Right, prototype for the high voltage supply/switch of the uTracer4.

When one starts thinking about the requirements for a next generation uTracer, before you know it you end up with a monster that is designed to do everything, but that has lost all the “Sense and Simplicity” of the original uTracer3. This problem has been hampering me over the past years in developing a new uTracer. Figure 1.1 shows some of the attempts for a new uTracer that I have been working on during the past years. Some of these attempts were published, such as the uTracer4, while other attempts never reached my webpages. Most of these attempts failed because the design became unmanageable complex losing all the charm of the original uTracer. Hoping to have learned from all these failed attempts, I decided to have a new go at a uTracer that offers improved performance, but still has the simplicity and elegance of the uTracer3.

Figure 1.2 The 6146B is a popular tube amongst radio amateurs. I made it the “target tube” for the uTracer6.

Sometime ago somebody gave me a couple of 6146B beam power tubes. Searching the internet I discovered that the 6146B is a very popular transmitting tube used by radio amateurs. On studying the 6146B data sheet I found that this is the kind of tube that would really need all the new features I have in mind for a possible uTracer6! Voltages up to 800 V, currents in excess of 500 mA, and both positive and negative grid biases. So I decided to take the set of curves of Fig. 1.2 as a kind of target for the uTracer6 specifications.

Based on the experience with the uTracer3, the never finished concepts for the uTracer4 and 5, as well as the experiments published in the Lab Notebook page, I think I have a feeling of what is reasonably possible, and from this I come to the following tentative uTracer6 specifications:

In the following pages we will investigate how realistic these specifications are. One thing that I have learned the past years is to make incremental steps. So both for the hardware, but especially also for the software I will stay as closely as possible with what is there for the uTracer3. As usual I will include some “personal memorabilia” in these pages, mainly for myself since these pages primarily serve as my own documentation and I like to be remembered of happy personal events that happened concurrent to the experiments/writing of this work.
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2. Higher voltages: generation

Increasing the anode and screen voltages of the uTracer is not so easy. First of all, because the maximum working voltage of electrolytic capacitors is fundamentally limited to around 450 V. Secondly, boost converters are not really ideal for generating very high voltages. This is because the maximum voltage that can be applied over an inductor is limited, but more importantly, boost converters require a switching transistor that has both a very low on-resistance as well as a very high working voltage, which are conflicting requirements.

More suitable are flyback converters. Flyback converters use a transformer to generate the high voltage. A very nice property of the flyback converter is that the switching transistor on the primary side “does not see” the high voltage on the secondary side, but only a voltage that is scaled down according to transformation ratio of the transformer. This makes the selection of the switching transistor much easier since on-resistance and output voltage now basically have been decoupled. If you want to know more about the basics of flyback converters have a look at a page that I wrote ages ago.

Unfortunately, flyback converters have one, for hobbyists, huge drawback: they require a transformer. Among hobbyists, including myself, transformers are not very popular. Either you have to make them yourself (…) or you have to rely on what is commercially available, which is usually not that much. However, some time ago I uncovered a cute little transformer that at first side might be perfectly suitable for experimenting with higher voltages for the uTracer. The transformer in question is manufactured by Würth with type number 750311486, and is available from Mouser with type number 710-750311692.

Figure 2.1 The 750311486 flyback transformer from Würth.

This little gem, which measures something less than a cube centimetre has four windings. A primary winding with an inductance of 80 uH, and two secondary windings, each with 10 times more windings than the primary winding. My idea was to use both secondary windings in series to generate voltages towards 1000 V. Figure 2.2 shows a simple first test circuit to see if a flyback converter around this transformer would fit into the uTracer concept. The circuit consists of an oscillator that generates 10us wide pulses at a repetition frequency of 10 kHz. On the secondary side, the two secondary windings feed two 100 uF / 450 V capacitors in series. A tricky pitfall of this circuit is the isolation between the two secondary windings. According to the specifications, the isolation between the primary and secondary windings is specified at 1500 V. However, this is the isolation between primary side and secondary side. It says nothing about the isolation between the two secondary windings. To make it for the transformer as easy as possible, I made an elaborate construction with D1 … D4 that ensures there is no DC potential between the two secondary windings. Only during the flyback phase, there is a very short moment when all diodes are conducting, and where there can be up to 900 V between the two windings.

Figure 2.2 Circuit used to test the 750311486 flyback transformer from Würth under conditions resembling the uTracer topology.

The circuit worked like a dream. In less than six seconds, the circuit charged the capacitors to 900 V! There was no sign of dielectric breakdown between the secondary windings. Note that at the primary side a simple IRFI540 transistor was used that is specified at 77 mOhm on resistance with a breakdown voltage of only 100 V.

Figure 2.3 Full blown test circuit that I use(d) to study circuit concepts to increase the working voltage of the uTracer.
On the perfboard there is a second “piggyback” perfboard that allows me to test different inductor/transformer configurations.

The next step was to try the flyback transformer in an actual uTracer setup. Figure 2.3 shows a hacked uTracer where the high voltage section has been replaced by a breadboard flyback implementation. The breadboard contains the high voltage section in a flexible and reconfigurable configuration, and a new high voltage switch (more about that in a future write-up). Because I don’t want to completely change the uTracer’s firmware, the flyback converter is driven in the same way as the standard uTracer, so with pulses of a variable length (0 to 50 us) with a repetition frequency of 10 kHz. The uTracer measures the voltage and stops pulsing when the voltage reaches the set point value. To my surprize and disappointment the circuit worked less well than I expected. The curves were raged, especially for voltages below 150 V. Considerable time was spent on debugging the circuit. At first I suspected the oscillations that are common to flyback converters and that are caused by the charge stored on the drain of the MOSFET that cause oscillations after the transformer has delivered its charge to the load capacitor. Adding snubbers and diodes did give some improvement, but the circuit still continued to perform unsatisfactory.

I sometimes have the bad habit to start experimenting straight away without thinking too much or making calculations/simulations first. I plead guilty! After the initial disappointment I made some basic calculations that gave some clues as to why things didn’t work as expected. The most basic equation for any boost or flyback converter is given in Fig. 2.4A. It gives the current in an inductor with inductance L when it is applied to a voltage source Vsupl for t seconds. It basically says, when an inductance is connected to a voltage source, the current through the inductor increases linearly with time and at a rate inversely proportional to the inductance. Let’s take the flyback transformer as an example (Fig. 2.4B). If we apply 20 V during 15 us to the 100 uH primary winding, the current in the primary winding will rise to 2 A, the maximum current before the core starts to saturate.

Figure 2.4 Basic equation(s) governing working of a flyback converter.

When T1 is opened, the magnetic energy stored in the transformer will be dumped in the capacitor. The initial current flowing in the secondary side simply follows from the fact that the energy initially stored on the primary side of the transformer, is now transferred to the secondary side (Fig. 2.4D). So with a ratio in turns of a factor 10, the initial current on the secondary side is 2*sqrt(0.1)= 0.63 A. Here we see one of the reasons why the circuit is not working as expected. The secondary current causes a large voltage drop over the secondary winding amounting to 0.63*62= 39 V! This not only interferes with the proper flyback operation at low output voltages, but it also gives rise to substantial dissipation as could also be felt by the temperature of the transformer.

Back to the good old boost converter

After a lot of playing and tweaking of the flyback circuit, some improvement in the performance of the circuit was obtained, but it was too much of a hassle; the elegance of the circuit was lost.

Playing with the flyback converter, I realized however, that in the boost converter concept the problem of a too high voltage over a single inductor could be solved by using two inductors in series! From the uTracer3 we know that this type of inductors can easily handle 450 V across their terminals. By placing two inductors with half the inductance value in series, the current will flow through both inductors and consequently the generated voltage, which is proportional to dI/dt, will by definition be equally divided over the two inductors, potentially boosting the voltage up to 900 V

Anode and screen voltages up to 900 V would be great of course, but the problem of a switching transistor that can handle 1000 V with a sufficiently low on-resistance and acceptable price and availability remained a problem. Fortunately, it appeared that since the uTracer3 the industry has made significant progress. Silicon Carbide transistors with incredible specifications have now become available at very acceptable prices. An example is the SCT2750NY from ROHM that is available from Mouser under part number 755-SCT2750NYTB. This 1x1cm “monster” has a breakdown voltage of 1700 V, and an on-resistance of only 0.75 Ohm. It can handle currents up to 6 A, a power dissipation of 57 W, and comes at a price of around 6 euros. Apart from its large TO-268-2L package, the only other drawback that I can think of is that for the device to handle substantial currents, a gate voltage of around 15 V is required.

Figure 2.5 900 V Boost converter module.

Figure 2.5 shows the basic circuit of the high voltage boost converter. The circuit is pretty straightforward. The gate of the SiC fet is driven by a dual gate driver circuit. I used an IR25600, but many other gate driver ICs (like the MIC4427) will do just as well. The 8 pin package contains two driver circuits so that one can be used for the anode section and the other for the screen section. Resistor R1 limits the gate current during switching to a safe value. The filter consisting of L1 and C1 smooths out the high current boost pulses, which helps to reduce noise in the system. Finally, the two 2.7 M resistors R2 and R3 ensure that the output voltage is equally divided over C3 and C4 even when is a small difference in leakage current in these capacitors.

Figure 2.6 To the left the modular test uTracer now with the boost converter installed. Note the large size of the SiC transistor! The picture on the right is just a small souvenir for myself of the holiday spent in Friensland in the North of Holland where we visited Foudgum, a tiny village in the North of Holland where the Dutch writer and poet Piet Paaltjens lived (summer 2019).

Figure 2.6 shows the same test board as in Fig. 2.3, but now equipped with the boost converter. The circuit worked like a dream, without any hitches! Note the enormous size of the SiC SCT2750 in comparison to the inductors! An added benefit of the circuit is that the saturation current of the 150 uH inductors is much higher than the 330 uH inductors in the uTracer3: 3 A vs 1.5 A. By simply increasing the length of the boost converter pulses to 35 us, this current is reached. Since the energy stored in an inductor is proportional to the current squared, this means that per boost converter cycle 4 times the amount of energy (charge) is transferred to the capacitor(s). In plain English, they charge much faster, especially for voltages in the lower range.

Figure 2.7 The new 100 uF / 500 V capacitors from Rubicon (datasheet).

I had just finished writing this article when I found that Rubycon is now offering 100 uF capacitors at a working voltage of 500 V! There are more 100 uF / 500 V capacitors available, but they are of the bulkier snap-in type. The new Rubycon capacitors have the same diameter (18 mm) and pin spacing (7.5 mm) as the capacitors used in the uTracer. Using 2 of these capacitors in series would in principle make a 1000 V uTracer possible! Unfortunately these capacitors have a very long lead time of 20 weeks (!!) and Mouser does not have then on stock. Still, I ordered a bunch of them, and they will be delivered in April.
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Coming up: the high voltage switch