1. Boost circuit diagram
As shown in the figure below, the step-down converter is composed of DC input power supply Vin, turn-on switch S, freewheeling D diode (unidirectional conduction), energy storage element L, output capacitor C and load R.
Second, the capacitor is separated from the straight through the crossing
The ideal capacitance can only pass through the AC component, flow through the capacitor current, and only the AC part can pass through;
3. The importance of output capacitance
In extreme cases, what happens when there is no output capacitor in the DCDC circuit?
When there is no output capacitor, the output cannot be regulated; Therefore, the output capacitance is crucial to play the role of filtering and stabilizing the output;
Of course, depending on the parameters of the circuit, it will have an establishment time, and it is possible that this establishment time is very long, and it will not be able to establish stability, which means that the output has been fluctuating, and it may get bigger and bigger, and finally the circuit may fail. If we want to have a stable output, we have to choose a suitable output capacitor.
Fourth, the output voltage of the Boost circuit
According to the parallel circuit:
Vo=Vc
Relationship between diode current, capacitive current, and output current:
ID=Ic+Io
Hence the ripple of the output voltage, i.e., the ripple on the output capacitor;
The capacitance "matches" the change in the inductor current;
There are both AC and DC on the diode, and the load only passes through DC, and the most suitable parameter is selected to let the AC be filtered out by the capacitor.
5. Inductor current composition
There is always a current on the inductor, and the current of the inductor it is a triangular wave (red). When the MOS transistor is closed, the anode of the diode is grounded, so there is no current on it (blue). When the MOS transistor is disconnected, the current on the diode is equal to the current on the inductor.
According to the capacitance characteristics C=Q/V, i.e.:
t=C*dV/dt
After connecting the capacitors in parallel, the AC part can pass;
The output current of a buck circuit is related to the current over the entire cycle;
Unlike the buck circuit, the output current of the boost circuit is only related to the current during the Toff disconnection, that is, the diode current;
The main reason: the location of the diodes is different.
The change in inductor current is a ripple current, and part of the ripple current is transmitted to the diode, and finally to the output capacitor and the load.
6. The source and direction of the current in two states
The capacitor acts as a voltage source during closure and the inductor acts as a load. We want the output voltage to change during closure and :Ic=Io
The capacitor stores energy as a load during disconnection, and the inductor satisfies as a voltage source: ID=Io+Ic
7. Inductance ripple current of Boost circuit
The ripple current of the inductor is a triangular wave, and the effective current as output is the green part.
According to i=C*dV/dt, the inflow and outflow of current on the capacitor cause the output voltage to fluctuate, that is, the ripple component;
The ripple voltage (fluctuation) across the capacitor is due to the change in the amount of charge across the capacitor, thus:
△V=QCOUT/COUT
Similarly, when steady state is reached, the closing and disconnection times in either switching cycle result in a change in charge across the capacitor: -△QCOUT, △QCOUT (equal at steady state)
△QCOUT is the change in charge at both ends of the capacitor.
At steady state, the amount of charge change across the capacitor is
△V=(IC*Ton)/COUT
Because during closure Ic=I0, therefore
△V=(I0*Ton)/COUT
By simplifying, you can get:
△V=(I0*D)/COUT*fswitch
I0 is the output load current, Ton is the on-time of the switch, and COUT is the effective output capacitance.
8. Output capacitance value
根据式△V=(I0*D)/COUT*fswitch变换可得:
COUT=(I0*D)/△V*fswitch
Where: I0 is the output load current, Ton is the on-time of the switch, and COUT is the effective output capacitance.
When we set a certain switching frequency, duty cycle, and require a certain output ripple voltage, according to the size of the load, we can get the size of the required output capacitance value;
A few details that differ from the BUCK circuit:
During the closure of the BUCK circuit, the output capacitor is charged, while during the closure of the BOOST circuit, the output capacitor is discharged;
During the closure of the circuit, the output voltage (ripple) increases, while during the closure of the BOOST, the output voltage (ripple) decreases;
9. Output capacitor current
The red color above is the diode current and the blue is the output current, and there is a DC component difference between the two. The corresponding output voltage fluctuates very little, and is stable at 5.4V at the thousandth and ten-thousandth levels.
If the output capacitance changes from small to large, the ripple voltage of the output voltage decreases from large to small.
10. Output capacitance selection
【Capacitance value and error】Through the previous analysis, a higher output capacitance value can reduce the output voltage ripple and improve the load transient response, the capacitance value C is taken: Cour = xp, and the device value error and temperature rise need to be considered, usually selected according to the derating of 30%;
[Dielectric] X5R or X7R is recommended, due to the poor capacitor temperature and DC bias characteristics of Y5V and Z5U, it is necessary to avoid the application of Y5V and Z5U capacitors in DCDC circuits.
The rated voltage should be greater than the actual maximum voltage (including ripple peak) at both ends, and leave a certain margin.
【ESR】ESR affects the amount of ripple of the output voltage, because there is a continuous ripple AC component passing through the capacitor, so in order to avoid efficiency loss, try to choose a capacitor with low ESR, because the capacitor value of X5R or X7R material has a wide withstand voltage and temperature range, X5R or X7R is recommended.