Technical Information Site of Power Supply Design

# FAQ

## MOSFETs

### Design/Operation

It says "1.5 V Drive"; can I substitute another existing device (1.8 V Drive, 2.5 V Drive)?
"1.5 V Drive" means that driving is possible down to a minimum gate-source voltage of 1.5 V. Therefore, a device for driving at 1.8 V or 2.5 V can be replaced. However, when applying the maximum voltage rating of ±10 V or higher across the gate and source, a device for 4 V driving should be used.
A current at or above the rated ID flows momentarily; how should I decide whether the device can be used?

There is a SOA (Safe Operating Area) for each product; if operation is within this area, it can be judged safe for use.
Example: When VDS = 20 V, Idpeak = 2 A, Pw = 100 μs → operation is in the Pw = 100 μs area, so use is possible.

When determining MOSFET thermal resistance, if the amount of heat is large and it is anticipated that a larger board area than indicated for the given conditions will be needed, how would I go about calculating this？
The Rth will need to be measured separately for each board to be used.

### Specifications/Standerds

What is avalanche resistance?
Upon startup, during shutoff and at other times, the rated voltage may be exceeded; the avalanche resistance (energy) is the allowed energy that, if not exceeded at such times, will not result in device failure.
(However, this is on condition that the drain current and channel temperature ratings not be exceeded)

## Diodes

### Specifications/Standerds

What is the difference between the average rectified current Io and the peak forward current IFM?
When 60 Hz AC is applied, the average rectified current value is called the average rectified current, and the peak current value is called the peak forward-direction current. The ratings for each of these are given as absolute maximum ratings for each product.
Is it alright to use reflow soldering with diodes?
All surface-mount products are compatible with reflow soldering. For details on mounting conditions and the like, please refer to the product specifications for each product. For pin-insertion devices, there are DIP conditions that should be followed.
Is it alright to hand-solder diodes?
All products support hand-soldering. However, depending on the package, the solder temperature conditions are different. Please refer to the product specifications for each product.
Most diodes are described using the average rectified current, but up to how many amps DC can they pass?
For each product, an Io-Ta graph and an Io-Tc graph like the following are provided on its web page; they should be referenced.
However, the characteristics can change depending on the mounting board, applied waveform, ambient temperature and so on, and due caution should be exercised.
About how great is the allowable dissipation of a Zener diode?

A graph like that shown here appears on the web page of each Zener diode product. As an example, if the product being used is in the UDZS series, the allowable dissipation is 200 mW at an ambient temperature of 25℃, and is 100 mW at 87.5℃.
However, the allowable dissipation varies depending on the mounting board, and appropriate care should be taken.

What is the reverse recovery time (trr)?
The time during which current flows in the reverse direction momentarily when forward-direction current is suddenly switched to the reverse direction is called the reverse recovery time (trr).

### General/Others

What is a TVS diode (ESD protection)?
This is a diode that absorbs overvoltages, static electricity (ESD) and noise, to prevent circuit malfunction and protect devices.

## Bipolar transistors

### Design/Operation

Regarding VI(on) and (off) of Digital Transistor.
VI(ON)Min.:
Refers to the minimum input voltage value of the area in which the digital transistor is turned on. Hence if the device is to be switched from the on state to the off state, the input voltage must be further lowered below this minimum input voltage value.
VI(OFF)Max.:
Refers to the maximum input voltage value of the area in which the digital transistor can be held in the off state. Hence if the device is to be switched from the off state to the on state, the input voltage must be raised above this maximum input voltage value.
What method is used to calculate the base current entering the internal transistor of a digital transistor?
The method is explained below, using a DTC114EKA as an example.

During operation of the digital transistor, a base current is flowing in the forward direction between the emitter and base (E-B) of the internal transistor, and so a forward voltage (approx. 0.7 V at 25 ℃) appears across E-B. Because a resistor R2 is connected between E and B of the internal transistor in the digital transistor, the 0.7 V is also applied to R2. Hence we see that a current IR2 = 0.7 V/10 kΩ = 70 µA is flowing in R2.

When the input voltage Vin is 5 V, the potential at the IN pin is 5 V and the internal transistor E-B voltage difference is 0.7 V, so that a voltage of 5 V - 0.7 V = 4.3 V appears across both ends of the resistor R1. Hence we see that a current IR1 = 4.3 V/10 kΩ= 430 uA is flowing in R1.

Therefore we see that a current of 430 µA = 70 µA = 360 µA is flowing in the base of the internal transistor.

In this way we can calculate the base current flowing in the internal transistor. In order to turn on the digital transistor adequately (that is, lower the output voltage Vo(on)), the output current Io and input voltage Vin should be adjusted such that the output current Io is less than about 10 to 20 times the base current entering the internal transistor. If the input voltage Vin is inadequate and a sufficient output current cannot be passed, use a digital transistor with a smaller input resistance R1.

When the temperature is 25°C, the emitter-base forward voltage is about 0.7 V, but if there is a change in the temperature, the forward voltage drops by about 2.2 mV for every 1°C rise in temperature, so that when the temperature is for example 50°C, the voltage becomes about 0.7 V - (50°C-25°C)×2.2 mV = 0.645 V. Conversely, if the temperature drops to -40°C, the voltage is approximately 0.7 V + (25°C-(-40°C))×2.2 mV = 0.843 V.
In this way, the forward voltage VF changes with the temperature, and so appropriate care should be taken. Moreover, the forward voltage of 0.7 V at 25°C is merely a representative value. The voltage may vary within the range ±0.1 V, and so again, caution is required.
In the case of digital transistors, there is variation of approx. ±30% in the values of the internal resistances R1 and R2, and so the worst case should be considered when calculating resistance values.
Thus there is some variation in the forward voltage and in resistance values, and so the above calculation methods should be considered to be no more than guidelines.
Can a voltage be applied in the reverse direction of the breakdown voltage across the collector and emitter?
In the case of an NPN transistor, the breakdown voltage when a positive voltage is applied to the collector with the emitter grounded is VCEO, described in the specifications (for a PNP transistor, the breakdown voltage when the collector is grounded and a positive voltage is applied to the emitter is VCEO).
The breakdown voltage in the reverse direction from this (for an NPN device, the case when the collector is grounded and a positive voltage is applied to the emitter) is about the same as the breakdown voltage across the emitter and base. The emitter-base voltage is normally about 5 to 7 V, and so we ask that devices be used with the collector-emitter reverse-direction at 5 V or less (if a voltage close to the collector-emitter reverse-direction breakdown voltage is applied, the hFE may be lowered, or other degradation may occur). If the collector-emitter reverse-direction voltage is 5 V or lower, the only current flowing will be on the order of a leakage current.

Similarly to the above devices, voltage of up to about 5 V can be applied in the reverse direction across the collector and emitter (OUT-GND) of a digital transistor as well, but when a resistor is present between GND and IN, a current flows through this resistor.

### Specifications/Standerds

How large is the base current maximum rating?
The maximum rating of the base current is 1/3 of the maximum rating of the collector current (in the case of a Darlington-connected transistor, 1/10).
Example: Case of 2SD2656
The collector current maximum rating is DC 1 A, and 2 A for pulses, and so the base current maximum rating is 333 mA for DC and 666 mA for pulses.
In the case of a digital transistor, it is sufficient to stay within the Vin rating appearing in the specifications, since the Vin rating is set such that the input current is within the rated value.
How much variation is there in actually measured hFE values?
The range of hFE values is shown in ROHM's specifications. For some products, both upper and lower limits are displayed, but for some part numbers only lower limits are indicated. Part numbers for which both upper and lower limits are indicated can have actually measured values that span the entire range. Those part numbers for which only lower limits are indicated often have actual values in the range within about several times the lower-limit value; for further details, please contact us.

### General/Others

Please give a brief rundown of digital transistors.
Digital transistors are bipolar transistors with resistors added.

Normal bipolar transistor With a resistor R1 (input resistor) added With a resistor R2 (E-B resistor) added
■About the resistor R1
・Role of resistor R1: To convert the input voltage into a current and stabilize operation of the transistor
Operation of a bipolar transistor becomes unstable if an IC or other voltage output is directly connected to the input (base pin) to cause operation under voltage control.
By inserting a resistor (input resistor) between the IC and the base pin to cause operation under current control, the operation can be stabilized.
(This is because the output current changes exponentially with respect to the input voltage, but changes linearly with respect to the input current.)
The resistance R1 of a digital transistor is an internalized version of this input resistor.

Here transistor operation is compared for voltage input and for current input.
Voltage control
Input: Emitter-base voltage VEB
Current control
Input: Base current IB
Measurement circuit diagram
Theoretical equations
Input-output characteristics
On observing the input-output characteristic, whereas under current control, shown on the right side, the output changes linearly with the input, it is seen that under voltage control, shown on the left, the output changes exponentially with the input. In other words, when under voltage control, even a very small change in input causes a large change in the output current, so that operation becomes unstable.
For example, in the graph on the right side, when the input current is doubled from 40 µA to 80 µA, the output current doubles from 9 mA to 18 mA; but in the graph on the left side, a slight change of 14% in the input voltage, from 0.7 V to 0.8 V, causes the output current to increase seven-fold, from 10 mA to 70 mA.
Here, a slight amount of noise in the input voltage can cause a large change in the output current, making such a device unsuited to actual use.
Thus a bipolar transistor is more stable under current control, and so the input resistor R1 is necessary in order to convert the voltage output from the IC into a base current. A digital transistor incorporates this R1 internally, and so is effective for reducing the number of components and the amount of space required.

■About the resistor R2
・Role of resistor R2: To absorb leak currents and prevent malfunction
The resistor R2 prevents malfunctioning of the transistor by dropping to ground any leak currents, noise and the like that have entered from the input side.
 Leak currents enter transistor base as-is, transistor turns on. All currents are dropped to GND, and no current flows to transistor base, so the transistor does not turn on. Any minute input currents are all dropped to ground, but if the input current becomes large, a part of the input current begins to enter the transistor base, and the transistor is turned on. Input current is small When the input current is small, all of the input current is dropped to ground, and the transistor is not turned on. (There is no malfunctioning due to leak currents or the like) Input current becomes large When the input current becomes large, a part of the input current enters the base, and the transistor is turned on. (The transistor enters the normal on state.) VR2=VBE<(forward-direction EB voltage ≒ 0.7 V) VR2=VBE>(forward-direction EB voltage ≒ 0.7 V)

## Overall

### Design/Operation

What kinds of things should be considered when the ambient temperature has changed?

The allowable losses (Pc) must be alleviated (derating) according to the ambient temperature (Ta). From the following graph, the power applied to the transistor should be reduced according to the ambient temperature.
Derating is also necessary in the SOA (Safe Operating Area); for details, please refer to "Before Using ROHM Transistor". Moreover, electrical characteristics, such as the input voltage (VBE, VI(on), VI(off)), hFE, GI and so on for bipolar transistors and digital transistors, fluctuate with the temperature. Designs should employ electrical characteristic curve graphs to ensure that no problems arise in operation even when the temperature changes. The same precautions should be taken for MOSFETs as well.

### Specifications/Standerds

What is an absolute maximum rating?
Absolute maximum ratings are always stipulated for semiconductor devices. These are defined as "a limit value that must not be exceeded even momentarily, or when specifications are indicated for two or more items, limit values that must not be reached simultaneously for any two items" (JIS7032). If an absolute maximum rating is momentarily exceeded, degradation or failure could occur, or even if operation immediately after is normal, damage could be sustained that shortens the device lifetime. Hence the system design should ensure that no maximum ratings are ever exceeded.
What is the thermal resistance Rth?

This is the thermal resistance value until heat generated by a PN junction (or Schottky junction) of a silicon device is dissipated; Rth(j-a) represents the thermal resistance value from the junction to the periphery, Rth(j-c) is the value from the junction to the case, and Rth(j-l) is the value from the junction to the lead pins. A graph like the following appears on the web page of each product, and can be used to find the junction temperature during use Tj.
However, the thermal resistance value varies depending on the size and materials of the board, land patterns and the like, and appropriate care should be taken.

Do you support lead-free and RoHS directives?
All products are lead-free and conform to the RoHS directive.
Is there a MSDS (SDS) for transistor/diode products?
Transistor and diode products are solid objects, and therefore MSDS creation and provision are not applicable.
Why use a halogen-free design?
In the past, mold resins that included bromine-based flame retardants conforming to RoHS directives were used, but in order to further reduce environmental impact, we will be using halogen-free resins.
Definition of ROHM halogen-free materials (within uniform material):
① Chlorine concentration of 900 ppm or lower
② Bromine concentration of 900 ppm or lower
③ Total concentration of chlorine plus bromine 1500 ppm or lower
④ Antimony trioxide concentration of 1000 ppm or lower
This conforms to the IEC61249 standard, satisfying the values sought by the major European manufacturers that have strict environmental requirements.
During solder mounting on a board by a customer, even if there is some Θ(theta) shift, self-alignment due to the surface tension of the molten solder is improved.
Pins themselves are shorter, and there is no longer a bending step, so that after mounting there are concerns that board bending stress and board mounting strength are lowered. However, absolute strength is sufficient even for flat lead-type devices, and there are no problems for practical purposes.
Can devices be used with the absolute maximum rating exceeded only momentarily?
Devices must not be used in such a way that the absolute maximum rating is ever exceeded, even momentarily. Breakdown and destruction of the transistor may occur, or there is the possibility of a lowered hFE or other degradation. In the case of single pulses, the safe operating area (SOA) should be checked to confirm the range of possible use. For continuous pulses, the device power and temperature must be calculated. For a detailed procedure for making such judgments, please refer to "Before Using ROHM Transistor" and "About TR Die Temperature". (In addition, also refer to sections relating to "Derating".)
In figuring MOSFET thermal resistance, if the packages are the same, can the thermal resistance between the channel and outside air be considered to be the same?
For small-signal products, they can be considered to be about the same.
However, for power devices, thermal resistance values may differ depending on the ratings, even when the package is the same.
Do semiconductor components conform to UL standards?
UL standards are safety standards. Transistors and diodes are not themselves UL-certified. However, mold resins for which inflammability is required use materials which have received UL94V-0 (fire-retardant) certification.
What is the difference between automotive products and general products?
For general-purpose products which have a proven track record in general consumer products, in essence there are no differences in characteristics, items assured in specifications, or materials.
In manufacturing processes, special management is executed so as to enable layer-specific handling.
Moreover, traceability for 15 years is assured.
Also, we try to accommodate any special individual requests from automotive application customers to the extent possible.

### General/Others

Is there anything to keep in mind regarding storage?
The following conditions are recommended for all products.
(1) If these products are stored in the following environments or conditions, there are concerns for performance degradation, soldering properties, and other performance aspects, and so storage in these environments and conditions should be avoided.
・Storage in places with saltwater air or an abundance of corrosive gases, such as Cl2, H2S, NH3, SO2, NO2, and the like
・Storage at other than the recommended temperature and humidity
(2) Solderability and other performances are assured for one year from the date of shipment by this company, but only when the above-described storage methods have been followed. (3) Recommended storage conditions: Temperature 5 to 40℃, humidity 30 to 80％
Are halogen-free products supported?
They are supported for small-signal packages (please consult us separately).
Are overcurrent protection devices (ICP series) being produced or sold?
The production and sales of overcurrent protection devices (ICP series) have been terminated. We apologies for any inconvenience.
Is there anything I should be aware of concerning bending device pins?
When bending pin wires, the pin wire should first be chucked as in the diagram below, and the wire then bent with the main body of the device on the other side of the chuck. However, bending pin wires more than 90° should be avoided. Also, pins should not be bent repeatedly.

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