TC78B002FNG
2013-10-17
16
8. Soft Switching
Soft switching is performed by changing the output PWM duty gradually when conducting phase switches.
The time of soft switching is determined by the voltage of VSOFT pin and that of VOFF pin.
T
soft
> T
off
T
soft
< T
off
Voltage of VSOFT > Voltage of VOFF:
Total term of soft switching (Tsoft) is determined by the time of prior hall signal (180°) and the voltage of
VSOFT pin. OFF term is provided during soft switching. The time of OFF term (Toff) is determined by the prior
hall signal (180°) and the voltage of VOFF pin. During OFF term, the state of the power transistor is in the PWM
OFF mode. Soft switching operates in the period other than the OFF term, and output PWM duty changes by 16
steps in maximum.
Voltage of VSOFT < Voltage of VOFF:
It does not have the term of soft switching operation which changes the duty, but it has the OFF term. OFF
term (Toff) is determined by the time of the prior hall signal (180°) and the voltage of VOFF pin. During OFF
term, the sate of the power transistor is in the PWM OFF mode.
When next edge does not occur though time of T1’ passes, last output state continues.
Conducting pattern is reset in synchronization with the up edge and the down edge of the hall signal.
So, waveform indicates non-contiguous every reset when hall signal is offset and in speed up/slow down mode.
HP-PM
OUT1
OUT2
T1
T1'
T
off
T
soft
T
off
T
soft
T
soft
①
②
③
⑭ ⑮
⑯
④
T
off
T
soft
T
off
HP-PM
OUT1
OUT2
T1
T1'
T
off
T
soft
T
off
T
soft
