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Bill Sloman
Guest
On Friday, July 24, 2020 at 4:38:55 AM UTC+10, John Larkin wrote:
The idea that you ought to use flyback in a scheme for getting 20W at up to 1000V strikes me as wrong.
The problem with flyback is that you have to dump your quantum of energy into the inductance at low voltage, then wait for the inductance to ring up to the desired output voltage - using up some of the energy in charging the coil\'s interwinding capacitance - after which you have to wait a bit longer for what\'s left of that energy to get dump into the high-voltage reservoir capacitor.
Whenever I\'ve tried to do it, I\'ve ended up with bulky inductors which I\'ve had to gap before they can store enough energy.
Forward converters - which do include centre-tapped inverter transformer systems (as in the Royer and Baxandall inverters) - don\'t have to store anything like as much energy.
The catch is that you need a fairly high turns ratio to get the voltage step-up (but pi/2 less with the Baxandall inverter) which means a custom wound transformer, and the high turns ratio always means a relatively high inductance secondary and a relatively low self-resonant frequency (even if you can break up the secondary into stacked banks, which can dramatically reduce the inter-winding capacitance).
On the other hand you end up with a tolerably compact transformer and something like 95% efficiency, which Jim Williams wrote a lot of application notes to gloat about (Linear Technology application notes AN45, AN49, AN51, AN55, AN61, and AN65).
And you don\'t have to wrestle with the complexities of an integrated circuit switching regulator chip - none of them ever do exactly what you want them to.
Making the output voltage variable takes an effort - I like the idea of pulse-width modulating the drive into the inductor in a Baxandall class-D oscillator, but I\'ve never made it work with a real circuit, though it simulates fine.
--
Bill Sloman, Sydney
The idea that you ought to use flyback in a scheme for getting 20W at up to 1000V strikes me as wrong.
The problem with flyback is that you have to dump your quantum of energy into the inductance at low voltage, then wait for the inductance to ring up to the desired output voltage - using up some of the energy in charging the coil\'s interwinding capacitance - after which you have to wait a bit longer for what\'s left of that energy to get dump into the high-voltage reservoir capacitor.
Whenever I\'ve tried to do it, I\'ve ended up with bulky inductors which I\'ve had to gap before they can store enough energy.
Forward converters - which do include centre-tapped inverter transformer systems (as in the Royer and Baxandall inverters) - don\'t have to store anything like as much energy.
The catch is that you need a fairly high turns ratio to get the voltage step-up (but pi/2 less with the Baxandall inverter) which means a custom wound transformer, and the high turns ratio always means a relatively high inductance secondary and a relatively low self-resonant frequency (even if you can break up the secondary into stacked banks, which can dramatically reduce the inter-winding capacitance).
On the other hand you end up with a tolerably compact transformer and something like 95% efficiency, which Jim Williams wrote a lot of application notes to gloat about (Linear Technology application notes AN45, AN49, AN51, AN55, AN61, and AN65).
And you don\'t have to wrestle with the complexities of an integrated circuit switching regulator chip - none of them ever do exactly what you want them to.
Making the output voltage variable takes an effort - I like the idea of pulse-width modulating the drive into the inductor in a Baxandall class-D oscillator, but I\'ve never made it work with a real circuit, though it simulates fine.
--
Bill Sloman, Sydney
