Spent some time braking apart old and not very old computer mainboards. Collected few kilos of capacitors and MOSFETS for future experiments.
Nice set. Press on image for big (600k) photo.
So if you need to repair some mainboard’s capacitors, you have lots of spares. 🙂
Today I spent some time breaking apart some TV transmitters. I was looking for power transistors. Opened one very strange module with heavy heat-sinks. This is the photo of transistors I found inside:
It is Russian made Gallium Arsenide power FET transistor mounted on heat-sink. Also some filters made on ceramics. Very nice and useless stuff unless your are making some ultra-high frequency transmitter (~3GHz). The prints on transistor are:
3П610Б-2, 3П910Ð-2, 3П910Б-2.
Alternative output connection.
In other post I used classic output connection- collector load. Browsing in the internet I found Russian schematics of electric shock gun. It is based on the TL494 chip. But the output was connected in different way- using emitters while collectors were connected to Vcc. Here is the part of the schematics:
On the testing board everything is working. Quite cool, as we can eliminate one or two transistors- not need to invert signal to mosfet. Why this type of schematics is not in datasheet? … On original Texas Instruments there is NO such connection. But on ON Semiconductors same device (but with much more bigger values on maximum ratings) datasheet this is the first schematics! So, RTFM twice 🙂
Typically in self made small hardware (DIY), power from mains transformed and rectified using simple wall “cube”. But this cube produces typically 12V. A bit too low for VFD. If I would like to use some higher voltage cube for 25V it will be hard to find, and to much power will be dissipated in 5V supply for logic parts. There is one chip in AT/ATX power supply: TL494 (or analog), pulse width modulator, generator, etc. etc. Switching power supply controller. Using this chip you can build any switching supply, or PWM regulator. Using typical parts from AT power supply and powerful mosfet from mainboard I constructed boosting regulator. Schematics are here.
Press on image of here to see the schematics.
The main parts are: T1, Q1, L1, D1. T1 is used to drive powerful mosfet. Mosfet is connected in simplified way, so called passive on. It must be driven using totem pole schematics, but who cares if it works. L1 is some inductor from old HP printer (about 50 turns, 1cm height, 0.5 cm width with windings, open inductor). D1 is fast schotky diode from some other device. It is SMD part from Motorola- the device is unknown. (Motorola just loves to encrypt devices…some top secret paranoia). The TL494 is connected in alternative way. It can be connected in typical way described in older posts.
While testing this device, do not overload, do not load asymmetrically. For checking your system, remove nixie and load secondary winding of transformer with simple resistor.
Download small (~500kb XVid AVI): with oscillograms. And now take a note, that on the “drain” oscillogram you see other phase pulses. Even when mosfet if in off stage, the induction from other primary winding cause voltage to appear. This may kill mosfet. And if load is unbalanced or not ideal, you can find lots of parasitic oscillations. So design engineer must take care of them. Parasitic oscillations can be killed with “snublers”- simple RC network on transformer windings. This devices takes some of the high frequency power and dissipates it.
And now lets look in internet:
Half-bridge and full-bridge |
Push-pull |
Forward converter |
|
minimum mosfet voltage rating |
supply voltage plus a safety margin
230VAC*1,414 + 50V |
double the supply voltage plus a large safety margin 2*230VAC*1,414+100V |
double the supply voltage plus a large safety margin 2*230VAC*1,414+100V |
other mosfet properties |
medium voltage 400V fets, so a <0.2 Ohm channel resistance is typical => high current, low loss |
high voltage 800V fets, >2.0 Ohm channel resistance is typical => low current, high loss |
high voltage 800V fets, >2.0 Ohm channel resistance is typical => low current, high loss |
mosfet body diode: |
must be disabled, otherwise mosfets explode if supply <50VDC: if supply >50VDC: |
can be ignored |
can be ignored |
realistic power levels |
many kW |
some 100W |
some 100W |
what limits the power level |
base-feed transformer core power handling capability (saturation, induced currents causing core heating) mosfet current ratings (paralleling more than two fets, the right way, mosfet switching and conduction losses |
primary leakage inductance, huge voltage spikes (up to kV range) at increasing power levels, makes use of snubber circuits imperative (=>high heating losses and low efficiency, and high circuit complexity) >=800V fets are expensive and can’t handle much current |
(same as for push-pull) |
transformer design |
needs only one primary transformer design non-critical |
needs two identical and well coupled primaries, critical design – requires skills! ;o) |
critical design, only one primary only the first quadrant of the ferrite cores’ B-H curve is used, i.e. |
base feed transformer volt-seconds (Vs) imbalance: |
full-bridge: minimal danger of saturation, Vs imbalance mainly due to slight differences in mosfet channel on-resistances half-bridge: if the primary has a series coupling capacitor, then Vs |
major problems with Vs imbalance as fully identical pri windings are almost impossible to make. The driver circuit absolutely must have pulse-by-pulse current limiting. |
|
tuneable down to DC / 0Hz |
yes, by using a primary series coupling capacitor |
no (short-circuit at 0 Hz) |
no (short-circuit at freq towards 0 Hz) |
problems |
grief with gate drive transformers or floating channel mosfet driver ICs or optocoupler-tweaking |
grief with mosfets constantly dying on overvoltage, gate drive noise |
(same as for push-pull) |
Everything is working with simple passive mosfet drive. But if we increase switching frequency or place powerful mosfet (or even several mosfets)? There is no load in gate circuit, but there is gate capacitance. And if we need fast switching we must charge and discharge gate very quickly. With passive drive there are limitations. We can use active charge-discharge circuit. Also this circuit uses inverter transistor for proper signaling:
Totem pole mosfet drive (push-pull)
Few comments about schematics. R5 is TL494 output load- anything about 1K. R1 is Q1 load, not critical- 1K. Q1 inverts signal from controller. R2 is used to prevent overload of totem pole bases. Q2 and Q3 are complementary pair. R3 are useful if are few mosfets on the output, only few ohms. Q1 and Q3- C945, Q3-A733. All transistors from old PC power supply. R4 – load simulation.
(C945- npn transistor, ~50V, 100mA, 200mW; A733- pnp, ~50V, 100mA, 200mW)
Connect pair of these totem poles and connect to transformer like in older post. You can notice small increase of the performance. Don’t overload. There are lots of parasitic oscillations in this circuit.
Now let’s make some high voltage! As the schematics is done on the breadboard, I can’t produce high power application. This is because of very long wires used in the experiment. Also there is no any heat sink on the power mosfets. Connect transformer to the output stage according this schematics:
Push-pull load
The primary windings of the transformer are identical windings with about 10 turns. The secondary winding is about 100. So the transforming ration is 1:10. If you put 10V to primary, you must get about 100V in the output. The core is made from ferrite. I used some middle sized core from PC ATX power supply.
Be careful, the output on the transformer in under high voltage. The current is very low and will not kill you. But you’ll get nice zapp. One more danger- if you place big capacitor on the output, it can store fatal energy value.
Nixie on the output.
Nixies are working from DC voltage. This nixie needed about 160V to start. (The power supply of the device is about 15V- small wall “cube”)
(Other parts used in schematics: PHB55N03LT, 500 and 1K resistors, IN539 diode, unknown value HV capacitor, ИÐ-14 nixie, C1685 transistors from old Panasonic TV, 1000mkf capacitor)
the output of TL494 is quite weak, and we want more power. So we need some powerful transistors attached. The most easy to use (and very easy to get- from old computer motherboards) are n-channel power mosfets. We must invert output of the TL494 because if we connect n-channel mosfet to output, in shut-down stage mosfet will be open and lots of current will pass threw then in DC. This may burn everything here… Also all PWM (pulse width modulation) will be inversed! So get generic npn transistor and connect according this schematics:
Output stage with power mosfet and passive drive.
The powerful mosfet is driven in passive on mode. It is not very good, but for testing purposes and low power are useful. R1 in the schematics are npn transistor load. Select it according max current the transistor can handle. R2 is the load of our power stage. In next experiments it will be replaced with transformer coil.
Now, let’s look at real oscillograms…
Sawtooth signal on the CT pin.
Soft start animation. Oscilloscope is connected to C1, TL494 output pin.
Soft start animation. Oscilloscope is connected to the drain of power mosfet.
Now let’s speak about the pins.
GND (pin 7) and Vcc (pin 12) are usual power pins. Max Vcc is about 40V.
RT (pin 6) and CT (pin 5) are used for setting clock (1 … 300kHz). The frequency of the clock is calculated using table from data sheet and changing resistor (1.8 … 500 kΩ) and capacitor (0.47 … 10000nF). Or the on-chip oscillator can be bypassed by terminating RT to the reference output and providing a sawtooth input to CT, or it can drive the common circuits in synchronous multiple-rail power supplies.
REF (pin 14) is 5V reference output. Low power output- 10mA.
Pin FEEDBACK (pin 3) is used to control the pulse width. Is the input is zero, the width is maximum, if it is ~5V or more (compared to REF) than output is disabled. Input must be less than Vcc.
Output vs Feedback
DTC (pin 4) is Dead Time Control. This pin is similar to feedback, but it is used to make soft start. Regardless to FEEDBACK it controls the width of the pulse. If it is more than REF (~5V) the output is disabled, if zero- output is maximum and controlled by FEEDBACK. Here is example:
Simple soft start
There is small difference in DTC and FEEDBACK. It can be understood by reading data sheet.
C1 (pin 8 ) is collector of output transistor #1.
E1 (pin 9) is emitter of output transistor #1.
C2 (pin 11) and E2 (pin 10) are like C1 and E1, but for the transistor #2.
The output is max 200mA on each transistor.
OUTPUT CTRL (pin 13) is setting the type of output. If it is high (=REF), the output is two phase. If input is GND, the output transistors are working in same phase, so you can can connect them in parallel and thus, get more output current.
The last pins are used in error control unit. They are used to detect various errors and make protections from short circuit and under-, over-voltage protection. I’ll describe them in the other post.
From time to time, in newsgroups, in www forums and in www pages, people speak about converting DC to DC, or DC to AC, charge-pumping, building inverters (step up, step down converters) for various devices. From high power car audio amp to handheld stazer. There are lots of schematics (especially in Russian web) using simple TTL logic, transistor and other devices. But why invent bicycle? There is special, very cheap and very easy to find, chip. It is TL494 (or any other analog). You can found it in any PC power supply. According data-sheet, TL494 is “PULSE-WIDTH-MODULATION CONTROL CIRCUIT”.
The TL494 incorporates all the functions required in the construction of a pulse-width-modulation (PWM) control
circuit on a single chip. Designed primarily for power-supply control, this device offers the flexibility to tailor the
power-supply control circuitry to a specific application.
The TL494 contains two error amplifiers, an on-chip adjustable oscillator, a dead-time control (DTC)
comparator, a pulse-steering control flip-flop, a 5-V, 5%-precision regulator, and output-control circuits.
The error amplifiers exhibit a common-mode voltage range from –0.3 V to VCC – 2 V. The dead-time control
comparator has a fixed offset that provides approximately 5% dead time. The on-chip oscillator can be bypassed
by terminating RT to the reference output and providing a sawtooth input to CT, or it can drive the common
circuits in synchronous multiple-rail power supplies.
The uncommitted output transistors provide either common-emitter or emitter-follower output capability. The
TL494 provides for push-pull or single-ended output operation, which can be selected through the
output-control function. The architecture of this device prohibits the possibility of either output being pulsed twice
during push-pull operation.
The TL494C is characterized for operation from 0°C to 70°C. The TL494I is characterized for operation from
–40°C to 85°C.
Before doing any useful device, I deeply recommend to examine how this chip works. This will be some sort of TL494 tutorial 🙂
Take your breadboard, prototyping board or anything you like and connect wires according this schematics:
TL494 test circuit
If everything is connected right and detail are similar to used in the picture, the schematics will be working. Leave 3 and 4 open. Use your oscilloscope to test is oscillator is working- on CT (pin 6) you must see saw. The output will be null. Now connect FEEDBACK (pin 3) and DTC (pin 4) to ground (GND). You must find square pulses on the output. In the next message I’ll explain the pins in more details.
TL494 in the breadboard
