Motaxe V Minthe application note states this is the minimum voltage between the Vb and Vs. Should I use applicatioh 1uF cap. Yes, my password is: Do you already have an account? Jun 30, For what its worth, by assuming that I can ignore capacitor leakage current and Vmin, I used the above expression and the values I found for a frequency of 50Hz. Jun 18, 9.

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Guide: Head of the Department Prof. II Table Of Contents III Abstract IV Chapter 1: Introduction A Mosfet driver allows a low current digital output signal from a Microprocessor or microcontroller to drive the gate of a Mosfet. The driver has level shifting circuitry and sometimes a bootstrap circuit to allow the use of cheaper N type devices on the high side. And hence a driver is used. The driver is used to provide the current and also drives the gates with an appropriate voltage level not high to avoid the risk of damaging the MOSFET but also high enough to produce a low drain source resistance when turned on.

Gate drivers can be provided either on- chip or as a discrete module. In essence, a gate driver consists of a level shifter in combination with an amplifier. In contrast to bipolar transistors, MOSFETs do not require constant power input, as long as they are not being switched on or off. As a transistor requires a particular gate voltage in order to switch on, the gate capacitor must be charged to at least the required gate voltage for the transistor to be switched on.

Similarly, to switch the transistor off, this charge must be dissipated, i. When a transistor is switched on or off, it does not immediately switch from a non- conducting to a conducting state; and may transiently support both a high voltage and conduct a high current.

Consequently, when gate current is applied to a transistor to cause it to switch, a certain amount of heat is generated which can, in some cases, be enough to destroy the transistor. Therefore, it is necessary to keep the switching time as short as possible, so as to minimize switching loss. Typical switching times are in the range of microseconds. The switching time of a transistor is inversely proportional to the amount of current used to charge the gate.

For typical gate voltages of approximately V, several watts of power may be required to drive the switch. When large currents are switched at high frequencies, e. Consequently, a transistor which is directly driven by such a signal would switch very slowly, with correspondingly high power loss. During switching, the gate capacitor of the transistor may draw current so quickly that it causes a current overdraw in the logic circuit or microcontroller, causing overheating which leads to permanent damage or even complete destruction of the chip.

Higher gate capacitance - Digital signals are meant to drive small loads on the order of pF. Higher gate voltage - A 3. BUT, the gate has capacitance that must be charged up when it is switched on, and discharged when it is switched off. During this time when the gate is charging up, the MOSFET is partially on, operating in its linear region, and dissipating power. In such situations, there is a need to use high-side drive circuitry alongside low-side drive circuitry.

Undoubtedly, the most popular such driver chip is the IR It have a floating circuit to handle to boostrap operation. IR can with stand voltage upto v offset voltage. Its output pins can provide peak current upto 2 ampere.

It can also be used to as IGBT driver. Gate voltage must be 10 V to 15 V higher than the source voltage. Being a high-side switch, such gate voltage would have to be higher than the rail voltage, which is frequently the highest voltage available in the system. The gate voltage must be controllable from the logic, which is normally referenced to ground. Thus, the control signals have to be level-shifted to the source of the highside power device, which, in most applications, swings between the two rails.

The power absorbed by the gate drive circuitry should not significantly affect the overall efficiency. Figure 2. This interval occurs when the transistor is being driven on or off, and the voltage across its gate-to-drain parasitic capacitor CGD is being charged or discharged by the gate driver.

It is usually specified in nanocoulombs nC. Looking at the datasheet for a MOSFET, the gate charge characteristic has a flat, horizontal portion shown in figure below. That is the so-called Miller plateau. It is useful in estimating the driving requirements, because it evaluates the voltage of the plateau and the required charge to switch the device.

Thus, you can calculate the actual gate drive resistor, for a given switching time. The circuit reduces the overall power consumed in the driver and thus reduces the power loss. This is particularly important for high-frequency driver operation to take full advantage, in terms of efficiency.

Many techniques have been proposed for driving semiconductor devices at high frequencies like resonant gate driving, which is most suitable for coping with the high-efficiency requirement of HFETs. Yasser Nour, Shimaa F. So ideally no current flows into the gate when a DC voltage is applied. However a very small leakage current flows to maintain the gate voltage and also during the switching periods to change and discharge the device capacitances.

Floating channel designed for bootstrap operation 2. Tolerant to negative transient voltage 4. Gate drive supply range from 10 to 20V 6.

Separate logic supply range from 3. Cycle by cycle edge-triggered shutdown logic Matched propagation delay for both channels Outputs in phase with inputs.

PDIP plastic dual inline package has a very small legs and gets near impossible to solder without specialized equipments like micro tipped soldering irons or hot airjet soldering. User can use any of the above packages according to the requirement and cost effectiveness. Below figure shows two packaging types of IR Figure 3. It has rugged monolithic construction.

Monolithic refers to that whole electronic circuit is built on a single chip. The table below shows the lead descriptions of IR The use of each pin is as mentioned. Pin 3 is the Vcc and is the low-side supply and should be between 10V and 20V.

Pin 9 is V. It is the logic supply to the IR The actual voltage chosen depends on the voltage level of your input signals. It is as shown in graph below. This means that it can be used for almost all circuits, since most circuits tend to have around 5V outputs.

Using other PWM controller, ICs will probably be powered off at greater than 10V, meaning the outputs will be higher than 8V when high. So, the IR can be easily used. If the V is down to about 4V, desired result is not obtained. Pin 13 is VSS and is for the logic supply ground. It seems that they are independent and could perhaps isolate the drive outputs and drive signals. However, it would be wrong.

They are the logic inputs. Pin 5 is VS. The output to HO — high or low — is not with respect to ground, but with respect to Vs. VS is the high side floating supply return. When high, the level on HO is equal to the level on V , with respect to V. The output on LO is with respect to ground. Pin 11 is SD, a shut down pin.

When this pin is low, IR is enabled — shutdown function is disabled. When this pin is high, the outputs are turned off, disabling the IR drive. The necessity and design of bootstrap circuitry is as explained below. A large enough capacitance must be chosen for C1 so that it can supply the charge required to keep Q1 on for all the time.

The higher the on time, the higher the required capacitance. Thus, the lower the frequency, the higher is the required capacitance for C1. The higher the duty cycle, the higher is the required capacitance for C1. Yes, there are formulae available for calculating the capacitance. However, there are many parameters involved, some of which we may not know — for example, the capacitor leakage current. For high frequencies like 30kHz to 50kHz, capacitance used is between 4. If an electrolytic capacitor is used, a ceramic capacitor should be connected in parallel with this capacitor.

The ceramic capacitor is not required if the bootstrap capacitor is tantalum. A tantalum electrolytic capacitor, is a polarized capacitor whose anode electrode is made of tantalum on which a very thin insulating oxide layer is formed, which acts as the dielectric of the capacitor.

The tantalum capacitor distinguishes itself from other conventional and electrolytic capacitors in having high capacitance per volume and lower weight.

R1 and R2 are the gate current-limiting resistors.


Application Notes






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