<?xml version="1.0" encoding="US-ASCII"?>
<!-- DFIN New ActiveDisclosure (SM) XBRL Schema Document - http://www.dfinsolutions.com/ -->
<!-- Creation Date :2025-11-05T18:41:02.1406+00:00 -->
<!-- Copyright (c) 2025 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
<xsd:schema targetNamespace="http://www.fdus.com/20251105" attributeFormDefault="unqualified" elementFormDefault="qualified" xmlns:dtr-types="http://www.xbrl.org/dtr/type/2024-01-31" xmlns:enum2="http://xbrl.org/2020/extensible-enumerations-2.0" xmlns:fdus="http://www.fdus.com/20251105" xmlns:cef="http://xbrl.sec.gov/cef/2025" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:us-gaap="http://fasb.org/us-gaap/2025" xmlns:dei="http://xbrl.sec.gov/dei/2025" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xmlns:xbrli="http://www.xbrl.org/2003/instance" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:cef-pre="http://xbrl.sec.gov/cef-pre/2025">
  <xsd:import schemaLocation="http://www.xbrl.org/2003/xbrl-instance-2003-12-31.xsd" namespace="http://www.xbrl.org/2003/instance"/>
  <xsd:import schemaLocation="http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd" namespace="http://www.xbrl.org/2003/linkbase"/>
  <xsd:import schemaLocation="http://www.xbrl.org/2005/xbrldt-2005.xsd" namespace="http://xbrl.org/2005/xbrldt"/>
  <xsd:import schemaLocation="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd" namespace="http://fasb.org/us-gaap/2025"/>
  <xsd:import schemaLocation="https://xbrl.sec.gov/cef/2025/cef-2025_pre.xsd" namespace="http://xbrl.sec.gov/cef-pre/2025"/>
  <xsd:import schemaLocation="https://www.xbrl.org/dtr/type/2024-01-31/types.xsd" namespace="http://www.xbrl.org/dtr/type/2024-01-31"/>
  <xsd:import schemaLocation="https://xbrl.sec.gov/cef/2025/cef-2025.xsd" namespace="http://xbrl.sec.gov/cef/2025"/>
  <xsd:import schemaLocation="https://xbrl.sec.gov/dei/2025/dei-2025.xsd" namespace="http://xbrl.sec.gov/dei/2025"/>
  <xsd:annotation>
    <xsd:appinfo>
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        <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default" xlink:type="simple"/>
        <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain" xlink:type="simple"/>
        <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member" xlink:type="simple"/>
        <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension" xlink:type="simple"/>
        <link:arcroleRef arcroleURI="http://www.esma.europa.eu/xbrl/esef/arcrole/wider-narrower" xlink:href="http://www.xbrl.org/lrr/arcrole/esma-arcrole-2018-11-21.xsd#wider-narrower" xlink:type="simple"/>
        <link:roleRef roleURI="http://xbrl.sec.gov/cef/role/SecurityOnly" xlink:href="https://xbrl.sec.gov/cef/2025/cef-2025.xsd#SecurityOnly" xlink:type="simple"/>
        <link:roleRef roleURI="http://xbrl.sec.gov/cef/role/N2" xlink:href="https://xbrl.sec.gov/cef/2025/cef-2025.xsd#N2" xlink:type="simple"/>
        <link:labelLink xlink:role="http://www.xbrl.org/2003/role/link" xlink:title="labelLink" xlink:type="extended">
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          <link:loc xlink:type="locator" xlink:href="fdus-20251105.xsd#fdus_OnA1000InvestmentAssumingAFivePointZeroPercentAnnualReturnMember" xlink:label="fdus_OnA1000InvestmentAssumingAFivePointZeroPercentAnnualReturnMember"/>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="fdus_OnA1000InvestmentAssumingAFivePointZeroPercentAnnualReturnResultingEntirelyFromNetRealizedCapitalGainsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">On a 1000 investment assuming a five point zero percent annual return resulting entirely from net realized capital gains.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="fdus_OnA1000InvestmentAssumingAFivePointZeroPercentAnnualReturnResultingEntirelyFromNetRealizedCapitalGainsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">On a $1000 Investment Assuming a 5.0% Annual Return Resulting Entirely From Net Realized Capital Gains [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="fdus_OnA1000InvestmentAssumingAFivePointZeroPercentAnnualReturnMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">On a 1000 Investment Assuming a Five Point Zero Percent Annual Return [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="fdus_OnA1000InvestmentAssumingAFivePointZeroPercentAnnualReturnMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">On a $1000 investment assuming a five point zero percent annual return.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="fdus_OnA1000InvestmentAssumingAFivePointZeroPercentAnnualReturnMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">On a $1000 Investment Assuming a 5.0% Annual Return [Member]</link:label>
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        </link:labelLink>
        <link:presentationLink xlink:role="http://xbrl.sec.gov/cef/role/N2" xlink:title="presentationLink" xlink:type="extended">
          <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain"/>
          <link:loc xlink:type="locator" xlink:href="fdus-20251105.xsd#fdus_OnA1000InvestmentAssumingAFivePointZeroPercentAnnualReturnMember" xlink:label="fdus_OnA1000InvestmentAssumingAFivePointZeroPercentAnnualReturnMember"/>
          <link:loc xlink:type="locator" xlink:href="fdus-20251105.xsd#fdus_OnA1000InvestmentAssumingAFivePointZeroPercentAnnualReturnResultingEntirelyFromNetRealizedCapitalGainsMember" xlink:label="fdus_OnA1000InvestmentAssumingAFivePointZeroPercentAnnualReturnResultingEntirelyFromNetRealizedCapitalGainsMember"/>
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          <link:presentationArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/parent-child" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="fdus_OnA1000InvestmentAssumingAFivePointZeroPercentAnnualReturnResultingEntirelyFromNetRealizedCapitalGainsMember" order="1" priority="2" use="optional" preferredLabel="http://www.xbrl.org/2003/role/terseLabel"/>
        </link:presentationLink>
        <link:definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/cef/role/SecurityOnly">
          <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain"/>
          <link:loc xlink:type="locator" xlink:href="fdus-20251105.xsd#fdus_OnA1000InvestmentAssumingAFivePointZeroPercentAnnualReturnMember" xlink:label="fdus_OnA1000InvestmentAssumingAFivePointZeroPercentAnnualReturnMember"/>
          <link:loc xlink:type="locator" xlink:href="fdus-20251105.xsd#fdus_OnA1000InvestmentAssumingAFivePointZeroPercentAnnualReturnResultingEntirelyFromNetRealizedCapitalGainsMember" xlink:label="fdus_OnA1000InvestmentAssumingAFivePointZeroPercentAnnualReturnResultingEntirelyFromNetRealizedCapitalGainsMember"/>
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        </link:definitionLink>
      </link:linkbase>
    </xsd:appinfo>
  </xsd:annotation>
  <xsd:element id="fdus_OnA1000InvestmentAssumingAFivePointZeroPercentAnnualReturnResultingEntirelyFromNetRealizedCapitalGainsMember" name="OnA1000InvestmentAssumingAFivePointZeroPercentAnnualReturnResultingEntirelyFromNetRealizedCapitalGainsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="fdus_OnA1000InvestmentAssumingAFivePointZeroPercentAnnualReturnMember" name="OnA1000InvestmentAssumingAFivePointZeroPercentAnnualReturnMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
</xsd:schema>
