[introduction]The article passes introductory ideal circuit and actual circuit to emulate by engineer of science and technology of dragon of ADI agential fine horse, review switch weaveform, check PCB position, use good emulation model, in order to affirm the switch when manostat component of electron of the spendable when frequency is taller.

The article passes introductory ideal circuit and actual circuit to emulate by engineer of science and technology of dragon of ADI agential fine horse, review switch weaveform, check PCB position, use good emulation model, in order to affirm the switch when manostat component of electron of the spendable when frequency is taller.

PART.01 Install inductance and output capacitor constant

The switch frequency of DC/DC converter is normally between 300~500KHz, but gradually the product that popular switch frequency is 1~2MHz, some can achieve 4MHz or taller. The size that converter of high frequency DC/DC can reduce output electric capacity and inductance in order to save circuit board space, this is small-sized and portable the basic function of kind of product, but they fall very hard however in taller switch frequency achieve lower voltage.

This we found use ADI LTspice an ideal circuit and test and verify its work. Following graphs (graph 1) place is shown, use circuit compose builds input voltage to be 12(V) , load current to be the step up converter of 10(A) for voltage of 6(V) , output. When using this ideal circuit, when observing when switch frequency is 2MHz and 400KHz, what calculate and use inductance and the constant that output capacitance to be worth metabolic extent quite, can check later whether can be identical character acquired when moving along with computational constant when circuit.

Graph circuit of converter of 1 step up

PART.02 Choose inductor

Before computational inductance is worth need is calculated first Duty comparing, can consult use next figures (graph 2) place shows formula. Among them Vo is output voltage, VIN is input voltage, VD is diode to voltage.

Graph 2 Duty is more formulary than computation

The inductance that next computational switch frequency is 2MHz (the inductance that L1) and switch frequency are 400KHz (L2) . Computational formula pursues as follows (graph 3) place is shown, among them 40% what Δ IL is load current. When switch frequency is 2MHz, inductance value is 1/5 about, the v/LIT all over the ground number of inductor decreases. This shows, the advantage of tall switch frequency is to be able to choose the component with inductance lesser appearance.

Graph 3 inductance computation is formulary

PART.03 The choice outputs capacitance

Use next figures subsequently (graph 4) place shows formula, computation exports capacitance worth, what the ESR here points to is the ESR that outputs capacitance, and output electric capacity is worth a decision by acceptability output ripple.

Graph computation of voltage of 4 output ripple is formulary

This we leave imitate ideal position the circuit of step up converter, spread out ESR for the formula of 0, can reach seek formula of output electric capacity, following graphs (graph 5) place shows:

Graph computation of capacitance of 5 output electric capacity is formulary

Calculated according to formula the outputs capacitance and 400KHz(Cout2) output electric capacity that switch frequency is 2MHz(Cout1) , cout1=18 μ F, cout2=92 μ F, switch frequency is accordingly taller, electric capacity is smaller. Whether can the inductor that can check computation to go out next through emulating and output capacitor obtain needs output voltage.

PART.04 Use LTspice undertakes operating checking

Use through LTspice test and verify next on graph (whether can be 12V obtained below the inductance of the calculative in circuit of converter of step up of the ideal in graph 1) and output electric capacity, switch frequency in 2MHz and 400KHz output and ripple voltage are identical.

Graph the circuit of step up manostat that 6 switch frequency is 2MHz and 4MHz

Following graphs (graph 7) place is shown emulate to output voltage, among them gules (the result that Vout1) is 2MHz for switch frequency, below 12V output voltage ripple is 149mV about, blue (the result that Vout2) is 400KHz for switch frequency, below 11.95V output voltage ripple is 145mV about. From these the result can see, chooses inductor and output capacitor do not have a problem, below the circumstance of the manostat that and be in has tall switch frequency, can choose the small cell such as around use inductor and capacitor.

Graph weaveform of 7 output voltage pursues

But need examination pursues as follows (all sorts of electric current that graph 8) place shows are undee, blue inductance electric current has low pressure to place the characteristic of rate, use input capacitance to have filter wave easily. Can seeing the electric current that has shed gules switch and the electric current that had shed green to output capacitance is to have press quickly place the weaveform of PWM electric current of rate, this kind controls the PCB distribution that places rate electric current quickly must pass careful design. Stem from emulation purpose this, we are used to 2MHz and 400KHz press identically place rate undertake emulation, but go up in true equipment, of 400MHz pressure place rate when may comparing 2KHz faster.

Graph all sorts of 8 electric current are undee

PART.05 Summary

The article passes emulation test, the specification is jumped over when frequency of processing manostat switch tall, the reciprocity that uses in manostat implement the size with capacitor is smaller. However tall switch frequency also is accompanying a challenge, for example switch loss increases, tall switch node is pressed place rate and tall electric current to press place rate, find very hard also appropriate MOSFET. MOSFET must be had enough guide lowly electrify block, reduce with utmost ground guide know loss and realize sudden switch. The placement of the component on PCB and wiring should consider the parasitism inductance of PCB, sensitive node should not allow noise to travel.

 

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