Understand the function of inductance

 

Inductance often is outputted to carry the L in circuit of medium LC filter wave for switch power source by understanding (C is among them output electric capacity) . Although such understanding are correct, but the design to understand inductance must the more thorough act that understands inductance.

 

In step-down changeover (the switch control of Fairchild model implement) , the one aspect of the matter of inductance is to join DC outputs voltage. Another end joins through switch frequency switch input voltage or GND.

 

 

In condition in 1 process, inductance can be passed (Gao Bian “high-side” ) MOSFET receives input voltage repeatedly. In condition in 2 processes, inductance receives GND repeatedly. Because used this kind controller, can use two kinds of means to realize inductance ground connection: Pass diode ground connection or pass (low margin “low-side” ) MOSFET ground connection. If be hind a kind of means, converter calls “ synchronism (Synchronus) ” means.

 

If change,the electric current that considers to cross inductance in indelicacy of these two condition again now is. In condition in 1 process, the one aspect of the matter of inductance receives input voltage repeatedly, another end receives output voltage repeatedly. To converter of a step-down, input voltage must compare output voltage is tall, because this is met,form on inductance to pressure fall. Contrary, in condition in 2 processes, the inductance one aspect of the matter that receives input voltage repeatedly so is gone to by join the ground. To converter of a step-down, output voltage is necessarily carrying, because this is met,form on inductance negative to pressure fall.

 

We use inductance electrify to press computational formula:

 

V=L(dI/dt)

 

Accordingly, the voltage that goes up when inductance is when (condition 1) , the electric current on inductance can increase; The voltage that goes up when inductance is when losing (condition 2) , the electric current on inductance can be reduced. If the graph shows 2 times,carry the electricity of inductance:

 

 

Pursue through going up we can see, the biggest electric current that has shed inductance adds the in part of electric current of switch summit summit for DC electric current. The graph also calls ripple electric current on. According to afore-mentioned formula, we can calculate a peak value electric current:

 

 

Among them, ton is condition the time of 1, t is switch cycle (the reciprocal of switch frequency) , DC is condition of 1 occupy sky comparing.

 

Warning: The computation above is hypothesis each yuan of parts of an apparatus (on MOSFET guide pressure fall, of inductance guide pressure fall or the Xiaoteji in asynchronous circuit of diode to pressure fall) those who go up pressure falling to contrast is inputted and output voltage is negligible.

 

If, the fall of parts of an apparatus cannot oversight, be about to make mathematical calculation with following formula:

 

Synchronous change-over circuit:

 

 

Asynchronous change-over circuit:

 

 

Among them, rs adds inductance to circle the block of line resistor for inductive resistor impedance. Vf is diode of Xiao Te radical to pressure fall. R is Rs adds MOSFET to guide electrify block, r=Rs+Rm.

 

The degree of saturation of inductance magnetism core

 

Pass already electric current of calculative inductance peak value, we can discover what to arise on inductance. Very easy meeting knows, increase as the electricity that carries inductance, its inductance quantity can be reduced. Because the physical character of magnetism core material decides,this is. It how much can inductance quantity reduce is very important how much can inductance quantity reduce: If inductance quantity is reduced a lot of, converter won’t work normally. Arrive greatly when the electricity that carries inductance the degree of inductance actual effect, right now electric current calls ” of “ saturated electric current. This also is the basic parameter of inductance.

 

Actually, the switch power inductance in change-over circuit always can have degree of saturation of soft ” of a “ . Want to understand this concept to be able to observe the curve of the electric current of inductance Vs DC that measures actually:

 

 

After electric current raises fixed rate, inductance volume won’t drop quickly, this calls saturated character of “ soft ” . If electric current increases again, inductance can damage.

 

Attention: Inductance volume drops to exist in the metropolis in a lot of kinds inductance. For example: Toroids, gapped E-cores. But, rod Core inductance won’t have this kind of change.

 

Had the character of this soft saturation, we can know why to can stipulate in all converter the smallest inductance below DC output electric current is measured; And because the change of ripple electric current also won’t affect inductance badly,measure. Hope in all application ripple electric current as far as possible small, because it can affect the ripple of output voltage. This namely why the inductance quantity that everybody always cares DC to output electric current to fall very much, and the inductance amount that can issue in electric current of ripple of the oversight in Spec.