Although the switching converter have high conversion efficiency, but at different load conditions the power will be wasted and result in efficiency reduction. The power consumption can be divided into three parts. The first part is the large current pass to power MOSFET, due to the pass of power MOSFET can be equaled to a resistor (RON), it will result to a power losses. This power consumption is also called conduction loss (PCON) and express as follows:
2
CON OUT ON
P
=I R
(1)The second part is switching on and off alternately of the power MOSFET, as a results, the gate parasitic large capacitor of power MOSFET alternately charging and discharging.
There is a big loss of the converter and this power consumption is also called switching loss (PSW) and express as follows: converter operating in no loading. Although there is no load at output, but the converter still
TABLE II. COMPARISONS OF CONVERTER TOPOLOGIES
Topology Buck converter Boost converter Buck-Boost converter
Conversion Ratio OUT
Conversion Type Only Step-down Only Step-up
D>0.5 doing Step-up D<0.5 doing Step-down
can regulate the output voltage, this moment the current consumption of internal controller is called quiescent current. And the system power loss (PSYS) is defined the multiplication of quiescent current and input voltage. The efficiency of DC-DC converter is defined the ratio of the output power and input power including the power loss can be expressed as follows.
out OUT OUT
100%
Contrarily, the main power losses of light loading condition are PCON and PSYS, the solution to reduce power loss at light loading is decreases the frequency of control signal. The best way to increase efficiency is changes the control modulation.
The most three basic controlling technology is PWM (Pulse Width Modulation), PFM (Pulse Frequency Modulation) and hysteretic control technique which is introduced in section 2.2.1, 2.2.2 and 2.2.3 respectively.
2.2.1 Pulse Width Modulation (PWM)
Operating with PWM control, the power MOSFET are controlled by a constant clock cycle, the PWM control waveform is shown in Fig. 7 [5] [6]. While the ramp signal is lower than the control signal, the PWM signal at high level; the ramp signal is higher than the control signal, the PWM signal changes to low level. The main modulation is change the width of every clock cycle by the control signal and the output voltage is determined by the duty ratio of the PWM signal.
About the power consumption of Pulse Width Modulation focus on the conduction and switching loss, total power loss is expressed as follows.
2 2
( )
SW CON OUT Duty GP GN IN SW
P
+P
=I R
+C
+C V F
(4)As shown in Eq. 4, operating at PWM control the switching frequency is constant but output current varies with loading. That is to say, the switching loss is invariable with load but conduction loss will increase with the output loading, as shown in Fig. 8.
Fig. 7. Pulse-width modulation waveform.
Fig. 8. Analysis of conduction loss and switching loss at Pulse-width modulation
2.2.2 Pulse Frequency Modulation (PFM)
Operating with PWM control, the power MOSFET are controlled by a vary frequency, the PFM control waveform is shown in Fig. 9[8] [9]. The on-time of PFM controller is constant width and off-time is variable with loading. By controlling the off-time of every
switching cycle can obtain different switching signal to achieve desirable output voltage.
Therefore, the smaller output loading can reduce the switching frequency.
About the power consumption of Pulse Frequency Modulation also focus on the conduction and switching loss, total power loss is expressed as Eq. (4). Operating at PFM control both the switching frequency and output current varies with loading. That is to say, the switching loss and conduction loss will increase with the output loading, as shown in Fig. 10.
Constant pulse width Frequency modulation Light loading
Heavy loading
Fig. 9. Pulse-frequency modulation waveform.
Fig. 10. Analysis of conduction loss and switching loss at Pulse frequency modulation
2.2.3 Hysteretic Control technique
The hysteretic controller is shown in Fig. 11[10], the main control method is generating a hysteresis window. By controlling the upper and lower boundary to regulate the output voltage, when the feedback voltage touch to the hysteretic upper boundary, the power NMOSPET will turn on and PMOSPET will turn off to discharge the inductor current and feedback voltage will decrease. At the same time, the hysteretic window will change to the lower boundary. While the feedback voltage touch to the hysteretic lower boundary, the power PMOSPET will turn on and the power NMOSPET will turn off to charge the inductor current and feedback voltage will increase. The hysteretic window which is calculating by superposition theorem can be expressed as follows.
The features of hysteretic controller are described as follows; firstly, the main control circuit is comparator and the error amplifier does not be used, so it is no problem about system compensation. Secondly, without using any clock generator, the switching frequency of hysteretic controller is generated by system itself. The following is the calculation of
2 1 2 1
1 2 1 2 1 2 1 2
( ) ( )
H uppper lower REF IN REF IN
R R R R
V V V V V V V
R R R R R R R R
= − = + − =
+ + + + (5)
feedback voltage variation, as expressed as follows.
The voltage VFBAVG is the average voltage of feedback, ideally is DVIN, the parameter D the duty ratio of buck converter. Because the hysteresis window variation (VH) equals to feedback voltage variation (△△△V△VVVFBFBFBFB). Combining the Eq. (5) and Eq. (6), the switching frequency can as expressed as follows.
By controlling the resistor R, capacitor C and the ratio of resistors R1 and R2 can defined the switching frequency, its very suit for high switching frequency design. Finally, because the output ripple has been defined, it can’t choose the low ESR capacitor to reduce output ripple.
Fig. 11. The block diagram of Hysteretic control technique
0 (1 ) 0