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利用电脑电源改,可上400-700V ,
How to modify a surplus PC power supply to produce a 700V or 400V high-voltage rail.
The completed prototype, highlighting the construction of the output filter (L4 & C9). The positive lead is threaded through a small toroid 5-6 times before being soldered to the rear of the output terminal. The capacitor is soldered directly across the positive and negative terminals as shown.Valve circuits are not yet dead. While transistors are undoubtedly superior in most applications, the valve still offers several unique advantages. This applies first and foremost to its use in power circuits.
There exists a substantial body of opinion that valves outperform transistors in high-quality audio amplifiers, especially in the power output stages. The seriousness of these claims is reflected in the fact that some very reputable manufacturers offer valve amplifiers at the top end of their audio range. For the home constructor, reasonable-quality valve audio amplifiers can be made for a modest outlay using designs available freely on the Internet. These amplifiers are generally based on an EL34 or KT88 valve pair in the output stage, with both valves being readily available in Australia.
Another common application for valves is in the output stages of RF power amplifiers. They will operate satisfactorily at frequencies of up to about 30MHz, delivering up to 50W per valve. Their main advantage over RF power transistors, apart from being somewhat cheaper, is that they are much more tolerant of fault conditions.
When tuning a new power amplifier design, parasitic oscillations are often encountered which can easily destroy expensive RF power transistors. The valve, however, will live to see another day. Valves are therefore much more suitable for experimentation in new designs.
Although valves are readily obtainable, one of the main problems in their exploitation is the lack of suitable power supply transformers. Both the EL34 and KT88 are rated at a maximum plate voltage of 800V, with supply voltages in the order of 500-600V needed to extract maximum power and linearity. However, the only readily available high-voltage power transformers are isolating transformers, which deliver 240V, and magnetron transformers from microwave ovens, which deliver 1500V or more.
Clearly, both of these are unsuitable for our application.
The easiest way around this is to modify the switchmode power supply of a personal computer (PC), as explored in a previous issue of SILICON CHIP (October 2003). The older AT power supplies are readily available and have now become a surplus item. They are designed to produce about 200-300W, which is in the right ballpark for our application. For little cost, they include a ready-made PC board and almost all of the components we need for a HV switching power supply.
Moreover, due to its high operating frequency, the switchmode power supply offers much better regulation and far less ripple than can be obtained from a traditional valve power supply based on 50Hz AC rectification and smoothing.
Fig.1: the power section of the modified high-voltage supply. Using the values shown, the output is a well-regulated 700V, suitable for driving two power valves. You can also build a 400V version by winding T1 & L2 accordingly and selecting alternate values for capacitors C1 & C2 and the R3-R5 divider string (see text).Basic considerations
At first, it would appear that getting a PC power supply circuit to operate at high voltages involves just a few changes to the procedure outlined in the previous SILICON CHIP article. In particular, the number of power transformer secondary turns would have to be increased and all diodes, capacitors, and inductors would have to be replaced with high-voltage types.
The resistive ladder used to sense output voltage would also have to be changed. However, after a few trials, I found that the switching power transistors did not last long and it soon became clear that getting the circuit to operate at 700V would entail a more substantial redesign.
The main problem is that the volts-per-turn ratio used in the secondary winding of a standard PC ferrite-cored transformer (operating in step-down mode) is about one turn per volt output. This means that 700 secondary turns would be required for an output of 700V.
And that’s where we quickly run into problems. The power handling capacity of a coil, without considering insulation, is almost directly proportional to its volume. For example, if we wish to double the output voltage produced by a transformer, we have to double the number of secondary turns, and thus the coil length. The resistance of the coil will also approximately double.
However, if the coil is to deliver the same power, the output current is halved so that the coil’s "ohmic" (I2R) losses are halved. To compensate for this, we can halve the wire’s cross-sectional area so that the overall volume occupied by the coil is unchanged. Unfortunately, a multi-layered coil operating at high voltages and frequencies requires insulation whose thickness increases roughly proportionally to the voltage. As a result, our coil does not follow the volume law.
In fact, it is almost impossible to fit a 700-turn winding with adequate insulation into the space available around the core of a standard transformer.
The reason for the large number of secondary turns is that the original PC supply uses a full-wave centre-tapped rectifier configuration, which requires twice the number of turns of a full-wave non-centre-tapped configuration. However, even a non-centre-tapped configuration causes problems.
For a start, it is difficult to fit even 350 turns in the space available around the core. Also, the bridge configuration has no "cool" end of the secondary winding, with both ends alternatively switched between ground and maximum voltage. This means that heavy-duty insulation needs to be used between the primary and secondary windings.
Another problem is related to the mode in which the PC power supply operates. It relies on varying the duty cycle of the rectified mains pulses applied to the transformer to control the output voltage. This means that the secondary rectifier and filter network must be designed to supply an output voltage dependent on that duty cycle. A simple capacitive filtering network is unsuitable, as it would charge to the peak secondary voltage regardless of duty cycle.
The way this dependence is normally introduced is to place an inductor of appropriate value between the rectifying diode and the capacitor, forming an LC filter. However, combining an LC filter with a bridge rectifier does not clamp the secondary voltage, allowing large spikes to appear across the primary during transient conditions.
Fig.2: the schematic of a typical control section based on the TL494 PWM controller. The only changes needed here are the removal of the over-voltage detection circuitry and the addition of an over-current indicator, based on Q7 and an LED.Voltage doubler solution
The schematic diagram in Fig.1 shows a solution to these problems. It’s based on a voltage doubler circuit fed by a relatively low secondary voltage, making the secondary winding easy to fit around the core. A filter inductor (L2) introduces the duty cycle dependence necessary for pulse-width modulation (PWM), while diodes D1 & D2 clamp the secondary voltage, thereby limiting voltage spikes across the primary. |
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发表于 2007-10-24 11:57
，所以还是用回了变压器供电。
