Op 29 april 2004 15:55:38 schreef Psycho:
Voor wat diene die vrijloop diodes in een hexfet?:/
Deze zijn zuiver parasiteir en gewoon aanwezig dus
Op 29 april 2004 00:04:11 schreef Xenobinol:
Evil... de mosfets worden niet in verzadiging gestuurd, maar in hun lineaire bereik gebruikt.
Quote van IRF zelf:
At 01/02/2002 01:33 AM we wrote -
IR power MOSFETS are designed for use in switching circuits. The trend is towards ever more efficient devices with minimal Rds to reduce conduction losses and smaller input and output capacitances to reduce switching losses. A MOSFET used in an linear circuit such as an audio amplifier needs a relatively high Rds as this means a shallow sloping output characteristic, the slope is Vds/Id = Rds. If the slope is too steep as occurs in the later parts with low Rds it is very difficult to maintain the gate bias to keep the device on the linear part of the output curve in class A, AB or B type amplifiers. Class D is OK with low Rds parts as this is effectivley using the MOSFETS as switches. You do not state what type of amplifier you intened to construct, but using a N and P channnel pair equates to Class AB.
Please also see FAQ# 213 & 214.
IR power MOSFETS are designed to be used as Switches. This does not preclude their use in linear applications but as the device technology advances to provide ever better performance as switches with lower Rds(0n) and gate charge figures, this makes the devices ever less suitable for linear applications. A lower Rds(on) equates to an output characteristic with a steep slope, which makes it difficult to maintain a suitable gate bias to keep the drain current operating point stable. As the drain current is deliberatly varied in response to an applied input voltage, this changes the dissipated power within the part, changing the junction temperature and thus the value of Rds(on)i.e the slope of the output characteristic. If the device is being used in audio amplifier applications, other than class D for which most are ideally suited, the frequency of Rdson variation will not be seen as a fast variation as the thermal inertia will average them out to produce an average value for Rdson. This is the value you would use to solve the thermal equlibrium equation to determine if the Tj achieved in the application will be within permitted limits or not.
As higher voltage MOSFETS have higher Rds(on) values and thus a more shallow output slope these will be more suitable for linear applications.
All our current Application Notes are written around switching operations and we have none dealing directly with your or similar questions, except for AN-928 which was written some time ago.
With N channel devices, as the MOSFET is turned on by taking the gate voltage positive with respect to the source, you will have to arrange a gate bias supply positive of +200V by around a maximum of 10V if you want to control the MOSFET completely between full on and full off. As in your application the required output voltage also has to go negative you will have to use a split supply so that the effective gate voltage range is 0V to +210VDC for full range control.