One, overview of circuit of MOS canal drive

 

In use MOS the canal designs switch power source when circuit of motor perhaps drive, most person meets those who consider MOS to guide electrify block, the biggest voltage, the biggest electric current, also a few people consider these factors only. Such circuit perhaps can work, but not be outstanding, serving as formal products plan also is unallowed.

 

1, MOS canal sort and structure

 

A kind MOSFET canal is FET (another kind is JFET) , can be become to increase by ** model or extinct model, p raceway groove or N raceway groove in all 4 sort, but increase what apply actually only model MOS of N raceway groove is in charge of and increase model canal of MOS of P raceway groove, mention NMOS normally so, or what PMOS points to is these two kinds.

 

As to why to use extinct model MOS canal, do not suggest to inquire into the root of the matter.

 

Increase to these two kinds model MOS canal, more commonly used is NMOS. The reason is to guide electrify block is small, and easy ** . The application of switch power source and motor drive is so medium, use NMOS commonly. In the introduction below, also give priority to with NMOS more.

 

There is parasitism capacitance to exist between 3 pin that MOS is in charge of, we do not need this, because ** craft is restricted,arise however. The existence of parasitism capacitance makes should bother when design or choosing drive circuit a few, but avoid without method, introduce in detail again behind.

 

Can see on graph of MOS canal principle, leakage pole is mixed there is diode of a parasitism between source pole. This calls body diode, in drive sensibility load (be like motor) , this diode is very important. By the way one, body diode exists in single MOS pipe only, in interior of integrated circuit chip do not have normally.

 

2, MOS canal guides a character

 

Guide the means that connect serves as switch, be equivalent to switch closing.

 

The character of NMOS, vgs is more than fair value to be able to guide, the circumstance when suiting to use ground contact of Yu Yuan pole (low end drive) , it is OK to want grid voltage to achieve 4V or 10V only.

 

The character of PMOS, vgs is less than fair value to be able to guide, the circumstance when suiting to use Yu Yuan pole to receive VCC (high-end drive) . But, although PMOS is OK very use as conveniently high-end drive, but because guide electrify block is large, the price is expensive, replace sort to wait for a reason less, in high-end drive, still use NMOS normally.

 

3, MOS switch runs a loss

 

No matter be NMOS or PMOS, guide after connecting, have guide electrify block exists, such electric current can be on this resistor disappear energy consumption, the energy that this part wastes is called guide a loss. The choice guides the MOS canal with electrify small block can be reduced guide a loss. Present small-power MOS canal guides electrify block is in Europe of a few fine long hair to control commonly, of Europe of a few fine long hair also have.

 

MOS is in guide when connect and ending, scarcely is finished in the instant. The voltage of two end of MOS has a descendent process, has flowed electric current has an ascendant process, inside this paragraph of time, the loss that MOS runs is the product of voltage and electric current, call switch the loss. Normally switch loss connects a loss to be gotten greatly than guiding much, and switch frequency is faster, the loss is bigger also.

 

Guide the product that understands instantaneous voltage and electric current is very old, the loss that cause is very big also. Shorten switch time, can reduce guide every time the loss when connecting; Reduce switch frequency, can reduce the switch number inside unit time. These two kinds of method can reduce switch loss.

 

4, MOS manages drive

 

With bipolar transistor photograph is compared, think to make MOS canal guides commonly do not need electricity, want the value with GS voltage fair prep above only, OK. This accomplish very easily, but, we still need rate.

 

Can see in the construction that manages in MOS, in GS, parasitism capacitance exists between GD, and the drive that MOS manages, be pair of capacitance actually charge discharge. To capacitance charge need an electricity, because charge to capacitance,the instant can regard capacitance as short circuit, instantaneous electric current will be so bigger. Choose / when design MOS manages drive the first those who want an attention is the bulk that can provide instantaneous short-circuit current.

 

Of the 2nd attention is, use generally at the NMOS of high-end drive, guide if grid voltage is more than source pole voltage,need. And the MOS canal of high-end drive guides the source when connecting extremely voltage and leakage extremely voltage (VCC) identical, so at this moment grid voltage wants bigger than VCC 4V or 10V. If be in same in the system, want to get bigger than VCC tension, be about special step up circuit. Very much motor driver is compositive charge pump, those who want an attention is to should choose appropriate outside receive capacitance, with getting enough short-circuit current goes drive MOS is in charge of.

 

The 4V that says above or 10V are commonly used MOS canal guide electrify is pressed, need to have certain surplus of course when the design. And voltage is taller, guide a rate is rapidder, guide electrify block also is jumped over small. Also have now guide electrify presses smaller MOS canal to be used in different domain, but in system of 12V car electron, general 4V guides used quite.

 

The drive circuit that MOS is in charge of reachs its loss, the AN799 Matching MOSFET Drivers To MOSFETs of company of OK and referenced Microchip. Tell about very in detail, did not plan to be written more so.

 

5, MOS canal uses circuit

 

The character with MOS the most remarkable canal is switch character good, be applied extensively in the circuit that needs electronic switch so, be like switch power source and motor drive commonly, also illume moves light.

 

2, present MOS drive, have a few special application

 

1, low pressure application

 

When use 5V power source, at that time if use traditional totem pole structure, because the Be of dynatron has what 0.7V controls to press,fall, bring about actual and final the tension that is increased on Gate has 4.3V only. At that time, we are chosen nominal the MOS canal of Gate voltage 4.5V is put in particular risk.

 

Same question also happens in use 3V the circumstance of other perhaps and low-pressure power source.

 

2, wide voltage application

 

Inputting voltage is not a fixed cost, it is met as time other perhaps factor and fluctuant. This changes the driving voltage that brings about PWM circuit to offer MOS to be in charge of is not stable.

 

To let MOS the canal issues security in tall Gate voltage, the buy inside canal of a lot of MOS the amplitude that stabilized voltage canal restricts Gate voltage forcibly. Below this kind of circumstance, exceed the voltage that stabilized voltage is in charge of when offerred driving voltage, can cause bigger static power comsumption.

 

In the meantime, if use resistor simply to divide the principle that press to reduce Gate tension, can appear when input voltage is taller, MOS canal job is good, and Gate voltage is insufficient when input voltage drop is small, cause guide not quite complete, increase power comsumption thereby.

 

3, double voltage application

 

In circuit of a few control, the 5V that uses a model partly logically or 3.3V number voltage, and the voltage with power part use taller even 12V. Two voltage use in all means to join.

 

This raises a requirement, need uses a circuit, let low-pressure side can control the MOS canal of high-pressured side effectively, at the same time the MOS canal of high-pressured side can be faced as much 1 and 2 in mentioned problem.

 

Below these 3 kinds of circumstances, totem pole structure cannot satisfy output requirement, and a lot of off-the-peg MOS drive IC, also did not include the construction that Gate voltage restricts it seems that.

 

3, relatively general circuit

 

Circuit pursues as follows:

 

Graph the 1 drive circuit that is used at NMOS

 

Graph the 2 drive circuit that are used at PMOS

Here does a simple analysis in the light of NMOS drive circuit only:
 

Vl and Vh are the power source of low end and high end respectively, two voltage can be identical, but Vl should not exceed Vh.

 

Q1 and Q2 comprised a totem pole that turns over buy, use implementation segregation, when ensuring two drive are in charge of Q3 and Q4 to won’t be the same as at the same time, guide.

 

R2 and R3 offerred PWM voltage fiducial, through changing this fiducial, can let circuit work to be compared in PWM signal weaveform steep straight position.

 

Q3 and Q4 are used provide drive electricity, when because guide,be being connected, q3 and Q4 have a Vce only relative to Vh and GND lowest pressure fall, this is pressed fall to have 0.3V left and right sides only normally, greatly the Vce under 0.7V.

 

R5 and R6 are feedback resistor, use at undertaking sampling to Gate voltage, the voltage after sampling passes Q5 to produce an intense negative feedback to the base of Q1 and Q2, restrict Gate voltage in a finite numerical value thereby. This numerical value can adjust through R5 and R6.

 

Finally, r1 offerred the limitation of base electric current to Q3 and Q4, r4 offerred the limitation of Gate electric current that is in charge of to MOS, namely the limitation of the Ice of Q3 and Q4. Necessary when can be above R4 paralell connection quickens capacitance.

 

This circuit offerred the character that be as follows:

 

1, with low end voltage and MOS of PWM drive high end are in charge of.

2, the MOS of the requirement of voltage of tall Gate of PWM signal drive that uses little scope is in charge of.

3, the peak value of Gate voltage is restricted

4, the electric current of input and output is restricted

5, through using appropriate resistor, can achieve very low power comsumption.

6, PWM signal is turned over. NMOS does not need this character, an inverter comes to the buy before can be being passed solve.

 

When designing portable equipment and wireless product, improve product performance, lengthening batteries working hours is two issues that design personnel needs face. DC-DC converter has efficiency electric current of tall, output electric current of big, static state is small wait for an advantage, special apply to for portable equipment power supply.

 

At present DC-DC converter designs a technology to develop main trend to have:

(1) high frequency change a technology: As switch frequency rise, the bulk of switch transformer also is reduced subsequently, power density also gets promoting considerably, dynamic noise should get ameliorative. The switch frequency of small-power DC-DC converter will rise to megahertz class.

 

(2) technology of low output voltage: As the ceaseless development of semiconductor ** technology, the working voltage of microprocessor and portable electron equipment is lower and lower, this can offer low output voltage with respect to the DC-DC transformer of requirement future in order to get used to the requirement of microprocessor and portable electron equipment.

 

The development of these technologies raised taller requirement to the design of circuit of power source chip. Above all, as switch frequency rise ceaselessly, raised very tall requirement to the function of switch component, the switch that must have circuit of corresponding switch component drive to be as high as megahertz class in order to make sure switch component is in at the same time works normally below frequency. Next, to the portable electron equipment of batteries power supply, the working voltage of circuit is low (it is with lithium battery exemple, working voltage 2.5 ～ 3.6V) , accordingly, the working voltage of power source chip is inferior.

 

MOS canal is had very low guide electrify block, disappear energy consumption is inferior, in current and popular efficient DC – MOS canal is used to regard power as switch more in DC chip. But the parasitism electric capacity that provides as a result of MOS is large, the grid electric capacity that usually NMOS switch provides is as high as law of a few skins. DC of frequency of this expensive to the design work – the design of circuit of drive of canal of DC converter switch raised taller requirement.

 

There is a variety of CMOS, BiCMOS to use the logistic circuit that starts step up construction oneself and the drive circuit that regard big look as sexual load in design of low voltage ULSI. These circuit are leaving regular job under condition of 1V voltage power supply, and can be in frequency of the job below the condition of 1 ～ 2pF can reach laden electric capacity a few this to about a hundred megahertz very. The article was used just about lift step up circuit oneself, designed a kind to have capacity of drive of large laden electric capacity, agree with step up of frequency of low voltage, tall switch DC – the drive circuit of DC converter. Electric roadbed is designed at craft of Samsung AHP615 BiCMOS and emulate test and verify through Hspice, in 1.5V of power supply voltage, when laden capacitance is 60pF, working frequency can achieve 5MHz above.