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Design IdeasDecember 19, 1996 |
The supply in
Figure 1
meets the high-voltage requirements of pulsed networks, ignition systems, and
flashlamps. You commonly use flyback circuits to provide low-power, high
voltages. In a flyback circuit, a transformer stores energy when the main switch
is on, and that energy transfers into the load when the switch opens.
Constant-frequency PWM controllers work well for fixed output voltages, but
often waste valuable charge time by running into current limit when the output
voltage is low and the duty cycle is high.
The circuit in Figure 1 compensates for the extra discharge time necessary at low output voltages by making the off time inversely proportional to the output voltage. This feature results in the ability to repetitively charge more than 1.5 mF of capacitance to 360V in less than 3 msec with 3.2A input, which translates to an efficiency of more than 85%. Figure 2a shows the output voltage vs time for multiple charge/discharge cycles.
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In Figure 1, Q1 implements what first appears to be a fixed-off-time current-mode scheme by holding the oscillator capacitor low during the on time of the switch (Q2). During the on time, the primary current ramps to a fixed peak current of 6A. The values of R1 through R3, and the LT1431 programmable reference determine this peak-current level. The switch then turns off, releasing the oscillator capacitor, which charges at a rate determined by Q3 and VOUT, thus realizing variable off time. The cycles continue, maintaining a constant 4 to 6A primary charge current as the output voltage rises from 0 to 360V. Figure 2b shows the primary current as VOUT passes through 200V.
T1 consists of a series of three secondary-output windings sandwiched between the two primary windings (Figure 1b). Additional shims of approximately 0.015 in. each, attached to the legs of the EFD30-3F3 core (Philips Components, Riviera Beach, FL), result in a primary inductance of 70 mH. In other words, this construction produces a gap between the two halves of the core. You can also ground the center leg to produce a 0.03-in. gap, which should accomplish the same result. T1's small size (0.6 in.3) can accommodate a continuous output power of 25W, continuous or pulsed load, with peak output powers of over 30W. Input current is limited to less than 0.4A with the output shorted. (DI #1951)
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