С чего бы вдруг то ?
Например берем РН н.о. Астро 93-3702, там стоит ИМС Motorola mc33099dw
Смотрим даташит и видим PWM с фиксированной частотой 280 КГц (Osc. Freq. = 280 kHz)
Цитата:
The 33099 is designed to regulate the output voltage in diode-rectified alternator charging systems common to automotive applications.
The 33099 provides a regulated voltage feedback system to activate the alternator field current in response to system load current. The output voltage is monitored by an internal voltage divider scheme and compared to an internal voltage ramp referenced to a bandgap voltage. The 33099 regulates the system voltage to 14.8 V for the DW suffix and to 14.6 V for the CDW suffix by generating a pulse width modulation (PWM) voltage waveform at the GATE of an external MOSFET to provide an average alternator field coil current as a function of the internal voltage comparison.
Древовидный вид