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Electronics - Designing with discrete semiconductors

Created; 12/10/2015, Changed; 26/04/2018, 25/04/2018

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There are many different transistors and a lot of those are the same dice (component) but selected for different parameters and available in a range of part numbers. Some types such as Bipolar Junction Transistors with symmetrical Emitter and Collector junctions providing bidirectional high voltage blocking but do not appear to be available now. Evidently they are made and PNP power transistor are  within many low voltage drop regulator IC's to withstand high (automotive standard) reverse voltage. Designing with transistors can give low cost solutions or solutions that fit in to small or odd shaped spaces. You should so more modelling than you would with an IC and their are tools but I use pencil, paper and calculator or a sheet.

Some solutions using transistors over a wide temperature and voltage range so require extensive modelling using specific transistor data as opposed to using general rule of thumb rules. For example much of the lighting in automobiles must be of a specific colour and intensity over a very wide temperature range and must also fail unambiguously if an LED goes open circuit but not dim so causing a partial failure. The circuit fit in a small space. These lo-cost robust applications are the most demanding to model.

High voltage driver for particle deflection vacuum tube applications; 

The deflection transistor circuit in this Practical Wireless (1972/1973?) constructional project oscilloscope used MJE340 transistors which are called medium power transistors and they are quite slow. I replaced them with TO5 package TV video driver transistors that were better but I learnt later that some small E-line Ferranti transistors such as ZTX458 might have done a better job without the excessive power consumption of my design. I used lowish value resistors as collector pull up to improve the speed. The driver would have needed to be moved to just behind the Cathode Ray Tube base to keep the board tracks and the stray capacitance down that would otherwise be significant.

I had considered using a valve amplifier output stage - there was a lot that I did not know at the time particularly understanding the reading of data in data books. I had purchased this 1970 edition data book and now see that a pentode valve would have had a tiny fraction of the miller capacitance that any transistor has (<<0.1pF) so the power demanded would have been resolved as well because it would not be necessary to overcome the components capacitance provided I placed the driver very close to the CRT base. There would be heater power of 2W per valve. Manufactures data books even old ones are invaluable to people starting a hobby. A pentode valve has two screen grids between the control grid and the anode and reduce ga capacitance by 20 fold I observe from the data book compared to a triode valve's 1-5pF.

I have criticised the circuit below then changed the circuit showing some of the steps.

I have introduced snubber networks (R+C) that dampen switching circuits previously and I have also introduced gate drivers. The circuit below has such damping but it is not so plainly shown but the transistor stage has a limited gain (proportional) and capacitance (integral) those parameters are also dealt with within the op-amp in order to make it unconditionally stable (for gain of 1 or more). 

The circuit and its application will require very good electromagnetic and electrostatic metal screening as well as distance kept between it and anything that could interfere such as a power supply. That is the same care issues that I mentioned for the oscilloscope.

Here are some candidate solutions;

1. Relay - simple and can switch in 1mS - relays suit High voltage or RF switching applications.

2. A variable power supply - you are going to need a high voltage source anyway but it must remain compensated over the voltage range which is nearly possible by over compensating the power supply and loading it with a resistive load. But there will be a minimum voltage that will not be controllable below.

3. There are some high voltage MOSFET types you may find a low current type, here is one; NDFPD1N150CG, On Semiconductors, N Channel, 100 mA, 1.5 kV, TO220. Note; ID=1mA at 1.2KV. This transistor would require a lot more power than the BJT transistor solution below.

4. Add a linear output stage or use a linear driver - this is a good strategy but in the case of high voltage say 1KV - 3KV would require a number of small transistors daisy chained. Exploring this further;

    • Small BJT transistors are available (basically the Ferranti part I mentioned above have been developed further and are also available in SMT  packages). The design will drive the base negative into break down so that the best speed and voltage is achieved based on Vcbo rating but not Vceo that would suffer miller capacitance and secondary break down.
    • Here are some candidate transistors, there are others but the appropriate characteristics are not specified;
a. The first circuit below is based on FET transistors that I had not discovered until later in this design process example. 

b. The second circuit below is uses the MOSFET option 3 above. 

The operational amplifier function blocks show alternatives (below AL-0011-01D). These circuits do not directly monitor the output voltage so if there is some electrode or gate current which there may be the electrode voltage will not be accurate.

c. The first operational amplifier block is the faster, simple and more predictable, (although their are more components than the second circuit)

d. The second example is more DC voltage accurate but the AC behaviour less predictable because the gate drain capacitance pushes the output in the opposite direction, due to inter-electrode capacitance, than the direction that the output is going to go in eventually. Increasing R4 to 50R or 100R may make the circuit less likely to cause overshoot but the issue may be trivial but it can be modelled or tried if it matters.


The second circuit on the right below looks like a differential amplifier but it is not but it is two drivers. I have drawn this with OrCAD Capture that is a very good for circuit editing and I copied mirrored horizontally and pasted what I wanted. I always create a sheet 0 with design Notes, Modification list of recent changes and a To-do list of thing I may do later or could do if other things arise or just things that need further optimisation. For example;

Notes;

High Voltage Amplifier


In the above the drive speed could be compromised because the circuit is not truly driving the transistors in common base or with the all the transistor bases driven 
negative when they are turning off. In that case miller capacitance effect could multiply Ccbo by 100 (hfe is up to about 100) and thereby increase the switching time to 5mS or somewhere between that and the 20-50uS calculated using a simple model. In this application such a lower speed would not be a problem.

The maximum reverse break down current is not given so I have used the test current given. I have also avoided secondary breakdown by driving the base negative or by using common base of the BJT and using a very low current so that the transistors can be used at higher voltages. Precaution has been included to ensure that the power supply dose not rise too fast in order that the voltage is shared between the transistors thereby preventing the circuit capacitance that would briefly cause the voltage to be shared unequally to occur.
High voltage driver for particle deflection vacuum tube applications.

Transistor selection; 
 On-semi
MMBT6520L PNP 350V 500mA; Vceo at 1mA, Vcbo at 100uA, Ceb=100pF and Ccbo=6pF 

 Fairchild
FJT44, NPN, 500Vcbo 100uA, 400Vceo 1mA, 300mA, Ccbo=7pF,

 Diodes Ltd
MMSTA92, PNP, 100mA, -300 Vceo 1mA, -300Vbo 100uA, Ccb = 6pF 
DXTP560BP5, PNP, -500Vceo 10mA, -500Vcbo -100uA, Ccbo 8pF
FMMT560, PNP, -500Vceo 1mA, -500Vebo 100uA, Ccbo 8pF
FMMT459, NPN, 450Vceo 1mA, 500Vcbo 100uA, 500Vcev, Ccbo=5pF, Vebo 7V 100uA.

ARITHMETIC;
 Driving low; i = C.dV/dT, dt = C.dV/i (per transistor stage)
25uS = 5pF x 450V / 100uA

 Driving High is a RC time constant will reach 66% at; 
T = R x C, T = 4 x 10^6 x 5 x 10^-12, 
T = 20 x 10^-6
20uS

Circuit and load capacitance has not been included. This will be significant. 

Maximum VCC; 11V = Vebo + 2.5Vref + 100uA x 15K

Maximum VH <1,300V = 3 x Vcbo = 3 x 450V

Time constant of op-amp - should be at least 1/5th for stability plus margin for transistor tolerance; 1/10th transistor stage (~2uS);

Simple T.C is; 40uS = 3M9 x 10pF

Transistor stage gain; 260 = 3900/15

Gain for 3 transistor stages = 780
 Therefore TC divided by 780 = 0.5uS approx.

Circuit gain; 510 = 3900/7.5
 The overall op-amp gain is 2/3rd's but will (should - safe bet) be stable partly because the AC gain is 1 (minimum for stability) due to feed back capacitors.

I have redrawn the circuit but as two drivers and added some capacitors to give high frequency filtering and to drive the Base junction more stiffly at high frequency and hence make the transistor switch faster;

To Dos;
* Check the Vcb I test 100uA - is there a pulse rating that can be used.
 
* Check that Vref can sink current? YES +-5mA 

* Move op-amp to outside the driver block so that a PCB can be designed.

* Add a low loop gain boost high voltage power supply.

I also include a list of things to do; The high voltage power supply could use a simple softly driven single transistor Colpits or a Hartley oscillator. A cheap and quite adequate design perhaps using an off-the-shelf cold cathode high voltage transformer and having a low loop gain provided by a simple proportional control loop.

High voltage power supply and any other power supply or component with magnetic fields should of cause be kept far away from the tube but the plate driver is intended to be fitted close to the vacuum tube in order to minimise capacitive loading.
The operational amplifier has been replaced with a faster general purpose amplifier. Although the op-amp improves the circuit voltage accuracy the circuit would be reasonably accurate without it. I have selected a moderately fast op-amp in order to drive the transistors quickly and quite hard to achieve maximum speed as well. The bandwidth of the op-amp stage is faster than the transistor stage also ensures circuit stability. 

For optimum performance do not drive the output too near either supply rail. This circuit is not designed with a good AC performance in mind;  A step change will not create a smooth voltage ramp but change as different transistor sections are used, And do expect cross over distortion when the driving polarity changes - this why I have not claimed that this driver to be an oscilloscope deflection circuit.

C4 and C7 do not look beneficial they do not block high frequency forward conducting noise injection into a C-B junction because the voltage across that junction would be much higher than any noise level. By comparison the B-E junction will be conducting and will rectify and amplify noise Cceb that may be 100pF if I look at figures for similar transistors - so 1pF may not help but it can be removed or changed but that value may still block high frequency noise?

Although the op-amp is close to the transistor circuit it is placed and duplicated at the top level diagram in order for the CAD tool to handle the A and the B parts of the dual op-amp and also for the designer to handle the decoupling for the op-amp easily.

In the diagram above the Identical block's R1, C4, Tr5 and  R5, C7, Tr3 can be repeated as required to achieve the voltage required. But the final stage does not require the capacitor that are at C4 and C7, these would slow the stage. In the new circuit the top transistors Tr1, 4 C-B junction do operate in a linear part of the transistor so noise would in any case injected here would not cause offset or distortion. The circuits becomes;

                  
My preferred way of working is to draft the circuit, review it and then work on the PCB layout returning the circuit to add decoupling filtering and anything else the arises. In that way a doubt about whether something is required such as decoupling is dealt with in context of the physical layout and without going back wards and forwards so much finding space for things. In that way the function can be visualised in shapes, Electromagnetic and Electrostatic fields as well.

Whilst doing the design arithmetic for this transistor circuit I looked again at FET transistors and finally found one or two that do the job well. Even so this BJT based solution is fine and if necessary a hybrid but with high voltage small fets could be used. It would be necessary to check that they are available to low volume users because they are made for high volume automotive industry. This is likely because the parts are stocked by RS and Farnell but care must be taken to ensure this is not an end of life stock - that judgement is almost guess work though. The part that I have since found are;
  • IXTY01N100 - Enhancement mode N channel, 1000V, 100mA, 200uA at 80% of Vmax - this is better than the MOSFET I used above (3).
  • IXTP01N100D - Depletion mode N channel, 1000V, 100mA, 250uA at 80% of Vmax - this is better than the MOSFET I used above (3).
  • BSP135 or BSS126  - Depletion mode N channel, 600V, >7mA, 10uA at Vmax, an avalanche diode S-D integrated so cascoding more than one stage to raise the voltage is possible - this meets everything I was looking for. The off leakage current of the BJT transistor selected is comparable. This is a good solution anyway if cascoding of many stages to increase the voltage is required. But if you were to cascode many FET transistors then gate protection would be required (there may be internal protection but it is not specified) by comparison the BJT inherently has protection that I have figured into the design.
In the next page I shall deal with compensation but from a control loop perspective. In a subsequent page I shall restate and develop PCB to PCB and other filtering.

Next page; Electronics - Control Loops