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“The increased power capabilities, ease of control, and reduced costs of modem power semiconductor devices compared to those of just a few years ago have made converters affordable in many applications. In order to clearly understand the feasibility of these new topologies and applications, it is essential that the characteristics of available power devices be put in perspective. [1]” Power semi-conductor devices can be classified into three groups, diodes, thyristors and controllable devices. In this practical a fast recovery diode and a MOSFET are investigated.
The experiment was conducted on a circuit designed to determine the switching characteristic of a semiconductor device, particularly a MOSFET.
Key steps included determining the MOSFET's on-resistance, measuring switching losses during on and off transients, and investigating commutation effects by altering the circuit configuration.
The practical guide stipulates that the drain current must not exceed 1A, thus to ensure this a simulation on LTspice done before the practical determined that with a voltage of 10,8V the drain current will be 0,99A which does not 1A.
The MOSFET was driven with the CMOS output of the signal generator, a 15 V square wave was used as the input to the circuit.
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It was again ensured that the current the drain does not exceed 1A. The Switch and off transients of the MOSFET were determined respectively. The reverse recovery current of the diode was expected to be observable in the switch-on transient of the MOSFET.
To investigate the commutation effects the jumper was removed from the circuit.
Removing the jumper creates an open circuit which forces the current to flow through the inductor. When the jumper is in the circuit it caused a shirt circuiting loop on the board which can be thought of as an inductor.
Apparatus:
The on-resistance of the MOSFET was determined to be 0.326 Ω with a drain current of 997.47mA and a voltage drop (VDS) of 325.48mV across the drain and source.
𝐼𝑑𝑟𝑎𝑖𝑛 = 997.47𝑚𝐴
𝑉𝐷𝑆 = 325.48𝑚𝑉
𝑅𝑑𝑠(𝑜𝑛) = 0.326 𝛺
The switch-off transient of the MOSFET showed a voltage drop (VDS) of 13.8V and a drain current (IDS) peaking at 47.6A, indicating significant switching losses calculated to be 427.155μW.
VDS= 23.8 – 10 = 13.8V
IDS = 23.8/0.5= 47,6 A (ttop) for switch OFF transient
IDS = 20A
V1(t) = (56,2/10ns) 𝑡 + 1
=5,62 ×109t + 1
I1(t)= (-27.6/10ns)t+47.6
= - 2,27×109t + 47,6
P1(t) = V1(t) I1 (t) – this is for the off state of the MOSFET
= (5,62 ×109t + 1) (- 2,27×109t + 47,6)
= -1,55112×1019t2 + 2,64752×1011t + 47,6
Switching losses:
=1t + 47,6) dt P(t)
𝑇 = 427.155μW
The switch-on transient highlighted the reverse recovery current of the diode, with a charge (Qrr) of 0.42 μC calculated from the area under the curve, demonstrating the diode's response during MOSFET activation.
The reverse recovery current can be calculated by finding the area under the curve must be calculated. To simplify this calculation two cursors were used for measurements. The recovery time is the difference between the time measured by cursor one and two and the value of the peak voltage is given by cursor one. From these measurements the charge is determined as expressed below:
𝑄𝑟𝑟 = (𝑏𝑎𝑠𝑒 × ℎ𝑒𝑖𝑔ℎ𝑡) = 𝑉𝑟𝑟 × 𝑡𝑟𝑟 = 0.42 𝜇𝐶
The presence of a jumper in the circuit resulted in a higher peak voltage due to reduced inductance compared to the scenario where the jumper was removed, showcasing the impact of circuit configuration on commutation behavior.
Practical Question: Why does the peak voltage increase?
When the jumper is in place it causes a short circuit to the longer path and forces the current to flow through the shorter path. It can be noted that when the jumper in place the peak voltage is higher than when the jumper is removed. When the jumper is in the circuit there is a lower inductance as compared to when it is removed, this is the cause of the difference in waveforms.
The obtained on-resistance of the MOSFET is 0.326 Ω while from the data sheet is 0.405 Ω. The difference in both values can be attributed to many different elements i.e., the conditions that 0.405 Ω was determined is not the same as the 0.326 Ω was determined, also because the circuits boards have been used each year there could be degradation to the electronics.
The measurement obtained for the switching losses proved to be satisfactory as they are in line with what is expected from theoretical knowledge. The values found for Power loss and the charge of the reverse recovery are reasonable values and can be reliable.
Effects of commutation were observed, and it was found that when the jumper is placed in the circuit a higher voltage was measured, when removed the voltage dropped.
The aim of this practical was achieved through practical implementation and research. The practical achieved expected values within few margins of error due to certain elements that can be explained. This practical is a good example that practical work and theoretical expectations do not always corelate to each other.
Analyzing MOSFET Switching Characteristics: A Practical Study. (2024, Feb 21). Retrieved from https://studymoose.com/document/analyzing-mosfet-switching-characteristics-a-practical-study
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