Though the digital oscilloscope is the most common instrument in the circuit laboratory, some functions may not be known very well, the computing function of the digital oscilloscope is one of them. In fact can be simplified and plugged and supported the analysis of the switch circuit to heat with the computing function of the digital oscilloscope. The application note of this page will recommend how to utilize the ondoscope to obtain MAX5976 heat and plug MOSFET power consumption and exact value of the load capacitor in circuit.

The computing function of the digital oscilloscope is one of the digital oscilloscope most interesting functions, can simplify and expand and plug and support the analysis of the switch circuit to heat. The computing function of skilfullying use the ondoscope can obtain the instant power consumption when load capacitor or MOSFET is being turned on and shut off, it is very meaningful that these parameters plug and support the switch circuit in designing and analyzing heat, without the ondoscope computing function, this kind of parameter can only be similar to estimating.

The application note of this page recommends how to utilize the ondoscope to measure heat and plug circuit MOSFET power consumption and exact value of the load capacitor.

Set up the ondoscope

Simplify, we demonstrate through the way that MAX5976 heat that Fig. 1 show plug the circuit, it is Built-in: to examine flowing MOSFET that function and tape drove forms an intact power switch circuit following test voltage transformer method is suitable for being plugged the controlling circuit by heat that the discrete component puts up too . Connect the ondoscope probe to heat and plug the circuit according to the illustrated way of Fig. 1, the ondoscope can be obtained and calculated the design signal, two pieces of voltage probe connect the import and export separately, is used for measuring MOSFET both ends pressure drop, the corrent probe is used for measuring the electric current flowing through MOSFET.

Ondoscope probe of Fig. 1 connects to MAX5976 or MAX5978 heat and plugs the circuit, the ondoscope is calculated with the test result.

This basic connection mode is suitable for the non- Integrated heat and plug the circuit too. It input test and it receive front of MOSFET respectively at the probes of output voltage and behind MOSFET of MAX5976 at inside, MAX5978 is externally positioned MOSFET ,The corrent probe wants the and circuit to examine and flow the resistance to connect in series. In order to measure flowing through the accurate current value of the on-off element, the corrent probe should be put after the input capacitance, position before the output capacitance.

MOSFET power consumption

Switching element usually n ditch way MOSFET Power consumption equal to drain /source voltage difference VDS And drain current ID Product. In Fig. 1, VDS is the difference of the passway 2 and passway 1, ID is resulted from direct measurement of corrent probe. Ondoscope Tektronix which we used? DPO3034 There is a specialized calculation passway, can pass the following Fig. 2 The menu is disposed.

Fig. 2 DPO3034 digital oscilloscope advanced to calculate menu may edit formulae directly

Through editing a formula briefly, realize the passway 1 difference of and passway 2 and is multiplied by the value that the corrent probe measures, thus obtain MOSFET power consumption. After heat plugs the circuit and can enable, the output voltage is close to the input voltage with the slope of dV/dt, flow through charging current of output capacitance ID of MOSFET For:

ID = COUT * dV/dt

Look on it as the output high voltage dc generator capacitance as 360 F, VIN =At the 12V, MOSFET turn on the intersection of ondoscope and the intersection of screen and sectional drawing if picture 3a to show twinkling of an eye. MAX5976 limits the surge current of the instant in 2A. Notice the power waveform is a slope dropped, begin with 12V * 2A =24W, drops to 0W when the output voltage is raised to 12V, heat plugs the circuit and regards constant current as the load capacitor to charge, this is exactly that we are desired.

The power waveform that such way measures can be used for judging whether MOSFET works in its safety operation area SOA ,Or estimate MOSFET forms thermal Wen Sheng according to the relevant chart of data information. Want little a lot of the assumed error directly according to surveying the waveform. In addition, even surge current and dV/dt as Fig. 3b shows are not all constants, the measurement waveform of the power consumption is still accurate.

MOSFET work in the circuit of 3a picture 1 of Fig. dawdles red waveform ,COUT =360 F, the surge current is limited in 2A.

Fig. 3b surge current and dV/dt are not all at the time of constants, the power consumption waveform measured is still accurate. There is no current limiter in the surge current here.

Fig. 4 can obtain the total energy consumption that MOSFET is starting stage in the circuit of Fig. 1 to the total mark of the power

If the ondoscope supports the total mark function, MOSFET real total energy consumption in any highly energy-consuming incident further appears. The newly-increased red curve of Fig. 4, in order to utilize function of the ondoscope total mark to calculate the energy information that MOSFET consumes.

As Fig. 3a shows, COUT is 360 F, the surge current is limited in 2A. Because the power is a triangular shape in MOSFET open 2ms, the energy that can calculate probably there is 24W/2 * 2ms=24mJ becomes hot. Can find out when MOSFET open to finish, ondoscope the intersection of total mark and energy that function calculate data 24mWs ‘ =24mJ !

This kind of function is still suitable for analyzing other transient incidents of MOSFET, such as shutting off, shorting out or over load. Such exhaustive power and energy data can use SOA and temperature behavior when the accurate analysis MOSFET transient incident happen.

Measure the load capacitor

The total mark function in the digital oscilloscope computing function can also be used for measuring heat and plugging the load capacitor in the circuit. Suppose that does not consider the impact on electric current of impedance of circuit, then the capacitance value of load means a volt of electric quantity changed of every change of voltage of electric capacity; And the electric quantity is the total mark to time of the electric current. So, the value of plugging the surge current of the circuit divided by output voltage to heat is ranked, the digital oscilloscope can be calculated the magnitude of capacitance value of load accurately. In Fig. 5a, heat plugs the control device and outputs three pieces of 10 F ceramic electric capacity. It is zero because of the voltage initialization value as denominator, so any actual meaning does not calculated the curve at the beginning. But after VOUT exceeds the dog days, the capacitance value calculated is probably 27 F. Can find out, though the function of the ondoscope has been already very strong, still can’t reveal the capacitance value unit correctly as we hope!

The test of Fig. 5b is the same as Fig. 5a, but increased a 330 F electrolytic capacitor at output capacitance, when MOSFET opens to finish, can see the ondoscope reveals the result happens to be 360 F, we are desired. Notice resistance load will reduce the precision of capacity measurement, because it will shunt and enter the capacitive electric charge. But to transient time analysis, the result is still very useful.

Fig. 5a acts as COUT =30 F hour, the output capacitance value that the circuit of Fig. 1 measures

Fig. 5b acts as COUT =30 F 330 F hour, the output capacitance that the circuit of Fig. 1 measures

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